(updated below - Update II)
In order for the US to permit citizens of a foreign country to enter the US without a visa, that country must agree to certain conditions. Chief among them is reciprocity: that country must allow Americans to enter without a visa as well. There are 37 countries which have been permitted entrance into America's "visa wavier" program, and all of them - all 37 - reciprocate by allowing American citizens to enter their country without a visa.
The American-Israeli Political Action Committee (Aipac) is now pushing legislation that would allow Israel to enter this program, so that Israelis can enter the US without a visa. But as JTA's Ron Kampeas reports, there is one serious impediment: Israel has a practice of routinely refusing to allow Americans of Arab ethnicity or Muslim backgrounds to enter their country or the occupied territories it controls; it also bars those who are critical of Israeli actions or supportive of Palestinian rights. Israel refuses to relinquish this discriminatory practice of exclusion toward Americans, even as it seeks to enter the US's visa-free program for the benefit of Israeli citizens.

As a result, at the behest of Aipac, Democrat Barbara Boxer, joined by Republican Roy Blunt, has introduced a bill that would provide for Israel's membership in the program while vesting it with a right that no other country in this program has: namely, the right to exclude selected Americans from this visa-free right of entrance. In other words, the bill sponsored by these American senators would exempt Israel from a requirement that applies to every other nation on the planet, for no reason other than to allow the Israeli government to engage in racial, ethnic and religious discrimination against US citizens. As Lara Friedman explained when the Senate bill was first introduced, it "takes the extraordinary step of seeking to change the current US law to create a special and unique exception for Israel in US immigration law." In sum, it is as pure and blatant an example of prioritizing the interests of the Israeli government over the rights of US citizens as one can imagine, and it's being pushed by Aipac and a cast of bipartisan senators.
Israel's religious- and ethnicity-based entrance exclusions of American citizens are so well-documented and pervasive that even the US State Department provides an official warning about it in its official travel advisory for Israel, noting:

Some US citizens holding Israeli nationality, possessing a Palestinian identity card, or of Arab or Muslim origin have experienced significant difficulties in entering or exiting Israel or the West Bank."
Friedman notes that the bill is specifically designed to protect "Israel's regular and arbitrary denial of entry to US citizens . . . in particular US citizens of Arab descent or US citizens viewed as sympathetic to the Palestinians". As the former Director of the US Office of B'Tselem, Mitchell Plitnick, explained this week, concern over Israel's discriminatory exclusions was heightened by Israel's refusal this January to allow an American teacher of Palestinian descent, Nour Joudah, to enter Israel to teach English in the West Bank despite her holding a valid visa. As Plitnick noted, "Israel, undoubtedly, is concerned that a reciprocal agreement would compromise its ability to bar not only Palestinian-Americans, but also pro-Palestinian activists, from entering the country."
To accommodate this desire to discriminate, Boxer, Blunt and Aipac are now attempting to create a special exemption for Israel from the requirement to which all other countries are bound, and by which the US will be bound vis-a-vis Israelis. More amazingly, the only purpose of this exemption from these US senators would be to allow Israel to discriminate against the citizens of the country these senators are supposed to represent. As Mike Coogan of the US Campaign to End Israeli Occupation wrote in the Hill this week, "given that Israel views the mere existence of Palestinians as a threat, the [Boxer/Aipac bill] would essentially codify Israel's discrimination against Palestinian-, Muslim-, and Arab-Americans into US law." Indeed, Aipac is not even attempting to pretend this exemption has a non-discriminatory purpose. He further explained:
According to off the record accounts, AIPAC officials told members of Congress that there would need to be flexibility on this legal requirement to accommodate Israel's ongoing discrimination against Arab- and Muslim-Americans who attempt to travel to Israel and the Occupied Palestinian Territories."

So brazen is this bill in the special favors it showers on Israel at the expense of American citizens that even normally loyal factions in Congress are balking. As Kampeas reported:
'It's stunning that you would give a green light to another country to violate the civil liberties of Americans traveling abroad,' said a staffer for one leading pro-Israel lawmaker in the US House of Representatives.
Stunning indeed, but unfortunately far from surprising. Coogan similarly reported:
"Numerous public reports and off-the-record accounts from legislators and staff signaled that the brazenness and late release of the Israel lobby's legislative demands blindsided both individual members and various committees. Provisions appeared tone deaf and legally problematic, even among Israel's strongest supporters. . . .
"Behind closed doors, members of Congress and legal counsel alike balked at the idea that Israel be allowed in the program but remain exempt from the reciprocity requirement. Attorneys for both individual members and committees privately advised that complying with the request would be a flagrant violation of certain US laws barring discrimination, and would undermine the US government's call for the equal protection of all its citizens traveling abroad."

Apparently, none of that is a concern for Barbara Boxer, Roy Blunt or Aipac. Protecting the equal rights of their own country's citizens quite obviously has little significance when weighed against the supreme mandate to serve the interests of the Israeli government. That's not hyperbole: how else can this bill be fairly described?
The bill, formally named the United States-Israel Strategic Partnership Act of 2013, now has a total of 18 co-sponsors. That includes 9 Democrats and 9 Republicans, perfectly symbolizing how bipartisan is loyalty to Aipac on Capitol Hill. Besides Boxer, the bill's chief sponsor, that list of co-sponsors includes such progressive favorites as Ron Wyden, Amy Klobuchar, Richard Blumenthal, and Benjamin Cardin, as well as reflexive right-wing GOP Israel supporters such as John Cornyn and Saxby Chambliss. Perhaps most disgracefully, one of the co-sponsors is Democrat Debbie Stabenow of Michigan, whose state boasts a large Arab-American and Muslim-American population: exactly the people who would be targeted by this discrimination from a foreign government which she is seeking to legalize.
Coogan notes that, even with 18 co-sponsors in the Senate, the bill has attracted an unusually low level of support for an Aipac bill, which typically passes quickly and without much resistance. Plitnick says that "it certainly seems like AIPAC reached a little too far with this bill" and notes that Coogan's reporting suggests "this is a sign that AIPAC's grip on Congress might be weakening". This all follows an article in the Forward suggesting that Aipac's possible attempts to have Israeli aid uniquely protected from the budget cuts mandated by "sequestration" could "deprive aid to Israel of its broader support in the foreign aid community" by creating resentment in Congress and in the country generally.
Indeed, as AIPAC itself notes in touting Boxer's Senate bill, it includes numerous other provisions to further bolster Israel's special status vis-a-vis US policy. The bill begins by reciting the standard narrative favored by the Israeli government: "the Government of Iran continues to pose a grave threat to the region and the world at large with its reckless uranium enrichment program and defiance of multiple United Nations Security Council resolutions." At a time when American citizens are facing severe budget cuts, the bill vows "to continue to provide Israel with robust security assistance". The bill accomplishes its pro-discrimination goal by mandating Israel's entrance into the visa-free program provided that Israel "has made every reasonable effort, without jeopardizing the security of the State of Israel, to ensure that reciprocal travel privileges are extended to all United States citizens'". That is the special exemption that no other country in the program is permitted: Israel, alone in the world, is not required to reciprocate for US citizens but merely will make "every reasonable effort, without jeopardizing the security of the State of Israel, to ensure that reciprocal travel privileges are extended to all United States citizens."
Despite the unusually tepid reaction in Congress, this fight is far from over. Aipac rarely if ever loses when it comes to bills they want Congress to enact. As Coogan notes, "even without a large number of co-sponsors, it could pass under unanimous consent or other rules used by members of Congress to stymie debate or give the impression that legislation has more support than is really the case."
Aipac and its supporters have long expressed righteous outrage at suggestions that they prioritize Israeli interests over US interests and those of American citizens. Yet it is hard to imagine a clearer or purer example of exactly that behavior than this pernicious bill. If you're a US politician finding yourself working to allow a foreign government to discriminate against your own fellow citizens - by vesting that foreign country with a right that no other country (including your own) has - then you're essentially broadcasting to the world that the interests of that foreign government take precedence over your own and over the equal rights of your own fellow citizens.

UPDATE

Somewhat ironically, as Kampeas notes, what long kept Israel out of the US's visa-free program were "concerns in Congress' Homeland Security and Intelligence Committees that granting visa-free access to Israel's Arab minority could pose a security risk to the United States." So what had previously prevented this deal was that the US was long driven by the same discriminatory mindset that is now driving Israel: we want to keep Arabs out of our country! Notably, the Boxer/Aipac bill accommodates only the Israeli concern about Arabs in their country, but not the identical US concern, as they provide this discriminatory exemption right only to Israel but not to their own country.

UPDATE II

To illustrate how central the concept of reciprocity is in foreign relations (and to seize the opportunity to highlight a story I love so very much): on Friday, the US announced it was banning 18 Russian officials from entering the US due to human rights violations; today, Russia, in response, announced a list of 18 US officials banned from entering Russia due to their participation in the US torture regime, including David Addington, John Yoo, and two former commanding generals at Guantanamo. The Russians did not hide the fact that they were driven by one consideration only: the principle of reciprocity.
In 2004, the US began photographing and fingerprinting upon entry to the country the citizens of various countries, including Brazil; in response, a Brazilian court ordered the Brazilian government to begin photographing and fingerprinting US citizens entering Brazil. I recall quite well that a separate line was then created at all Brazilian airports under a huge sign that read: "for US citizens", where all arriving Americans waited in a long line. It's likely that the Brazilian government - which had no real interest in fingerprinting people - threw the fingerprints and photographs away. They did it for one reason: reciprocity.
This is the crucial, central principle which Barbara Boxer, Aipac and friends are discarding in order to benefit Israel. And what's most amazing is that they are discarding it not to the benefit of their own country and its citizens, but rather to their disadvantage, in order to benefit a foreign country. What they are saying, in effect, is that they want to waive reciprocity so that Israeli citizens can be treated better than US citizens in relations between the two countries. It is hard to overstate just how extraordinary that is.