Senators Bernie Sanders and Dianne Feinstein sent a letter on Thursday to Senate leaders urging them not to promote a controversial piece of legislation that will penalize boycotts against Israel and Israeli settlements in the West Bank.

Sanders and Feinstein, two of the most prominent Jewish members of the Senate, warned that the proposed legislation, called the Israel Anti-Boycott Act, will harm free speech in the U.S.

“While we do not support the Boycott, Divestment and Sanctons (BDS) Movement, we remain resolved to our constitutional oath to defend the right of every American to express their views peacefully without fear of actual punishment by the government,” Sanders, an independent, and Feinstein, a Democrat, wrote in their letter, which was sent to Democratic Minority Leader Chuck Schumer and Republican Majority Leader Mitch McConnell.

Both Schumer and McConell support the anti-boycott bill, which is being promoted by the pro-Israeli lobby group AIPAC.

The legislation has been met with opposition from leading civil rights groups, including the American Civil Liberties Union, over concerns that it will limit American citizens’ freedom of speech.

AIPAC and key Senators sponsoring the legislation have rejected these concerns by claiming that the bill will only punish large-scale companies that join the internationally-led boycott of Israel, and will not harm private citizens who choose to boycott the country.

The ACLU has warned, however, that the bill could be used for punishing private citizens, and would encourage states across the U.S. which have passed similar laws against BDS, to prosecute individuals for their political views on Israel.

Federal courts in Kansas and Arizona have ruled two such laws as unconstitutional over the past year, as Sanders and Feinstein noted in their letter.

“The courts’ reasoning in both cases applies with equal force” to the national anti-boycott law, they said.

Sanders and Feinstein also warned that the legislation will create de-facto American support for the settlements, by punishing American citizens who choose to boycott them.

“At a time when the Netanyahu government is pursuing policies clearly aimed at foreclosing the two-state solution, it is deeply disappointing that Congress would consider choosing to penalize criticism of those policies,” they wrote.

The impetus for the letter is an attempt several senators, including Maryland Democrat Ben Cardin, to include the controversial “anti-boycott act” within a broader budget bill that must pass before the end of 2018.

If the anti-boycott law is not included within that bill, it will have to be reintroduced the next time Congress convenes in January.

Ever since President Trump’s victory in the 2016 election, Sanders - who ran for president two years ago but lost to Hillary Clinton in the Democratic primaries - has become one of the most active and prominent voices on Middle East policy for the party.

Last month he successfully led, along with Democratic Senator Chris Murphy, a vote challenging the Trump administration’s policy in Yemen.

Sanders has repeatedly called on the administration to promote a two-state solution to the Israeli-Palestinian conflict and to work towards solving the humanitarian crisis in Gaza, which he believes poses a security risk to Israel.

Jewish Democrats are divided over the legislation, with a visible rift between mainstream traditional Jewish Democrats and the more progressive left-wing chapter of the community.

The Jewish Democratic Council of America (JDCA) announced its support for the legislation early on. “The Israel Anti-Boycott Act is consistent with the 2016 Democratic Party Platform that states Democrats ‘oppose any effort to delegitimize Israel, including at the United Nations or through the Boycott, Divestment, and Sanctions Movement’ and is also aligned with JDCA’s platform of opposition to global BDS,” Halie Soifer, Executive Director of JDCA said in a statement.

The Anti-Defamation League also publicly supported of the law, although a recently revealed two-year old memo showed the organization has debated the issue internally, with some of ADL leaders arguing that anti-BDS laws are “ineffective, unworkable, unconstitutional, and bad for the Jewish community.”

J Street, “the political home for pro-Israel, pro-peace Americans,” has officially voiced opposition to the bill, on the grounds that it “would do nothing to help strengthen Israel’s security or effectively combat BDS,” saying it stifles free speech and “erases the distinction between Israel and the occupied West Bank.”