WASHINGTON — Democrats on Capitol Hill are reexamining their support for a bipartisan bill intended to fight boycotts against Israel and the settlements after the American Civil Liberties Union warned U.S. senators last week that it endangers free speech in the United States and could lead to citizens’ going to prison simply for expressing a political opinion.

Two Democratic staffers who are involved in discussions over the legislation told Haaretz that over a dozen Democrats in both houses of Congress have already began to reconsider their positions in light of the letter the ACLU sent to U.S. senators. One of the first members of the House of Representatives to publicly announce such a review is Rep. Joe Kennedy III, a Democrat representing Massachusetts. Kennedy, a grandson of the late Sen. Robert F. Kennedy, is considered a rising political star in the Democratic Party and is one of 240 co-sponsors of the bill in the House.

The controversial Israel Anti-Boycott Act was proposed in the both the Senate and the House in March, but it only garnered national attention after the ACLU issued the letter last week expressing strong opposition to it. The leading civil rights organization warned that under the bill’s current language, U.S. citizens could face fines of hundreds of thousands of dollars, and potentially even go to prison for up to 20 years, simply for expressing support for boycotts of Israel and the settlements.

Another organization that came out against the legislation last week was J Street, which opposes boycotts of Israel and the boycott, divestment and sanctions movement but warned that the bill caused “serious concerns” because of its potential for restricting free speech. J Street also noted that the legislation does not differentiate between boycotts targeting the entire State of Israel and those aimed specifically at settlements in the occupied West Bank.

Criticism of the bill has not been limited to the political left. The American Interest, a magazine that is considered to have a centrist, pro-Israeli editorial line, wrote last week, “It’s true that the bill is probably not as blatantly unconstitutional as some of its opponents are arguing: it modifies a decades-old law that has been upheld in court, and legal scholars are still debating exactly how it would be enforced. But it’s not free-and-clear either. It seems possible, for example, that a person funding a student campaign for a university to enforce a UN or EU-backed Israel boycott could be exposed to criminal liability. Now, in fact the courts would quickly throw a case like that out — the First Amendment easily trumps a piece of feel-good legislation that Congress whooped through to gain popularity points. But it is hard to base a case for supporting a legal proposal on the argument that some of its obvious applications to domestic circumstances would be dismissed out of hand.”

These warnings have caused two of the bill’s original co-sponsors, Sen. Ben Cardin, a Democrat from Maryland, and Rep. Rob Portman, an Ohio Republican, to publish a letter on Friday responding to accusations that the bill would endanger free speech. They wrote that the bill’s critics misunderstood its’ language and that despite the ACLU’s warnings, no U.S. citizen will face legal penalties for supporting a boycott of Israel under the new legislation.

The two congressman explained in their letter that the most controversial part of the bill — the one detailing the criminal penalties for participating in boycotts of Israel — was in fact an expansion of a law, enacted in 1977, prohibiting U.S. companies from taking part in state-led boycotts of Israel. That bill was adopted in order to counter the Arab boycott of Israel. The new bill adds a new component to it, stipulating that the penalties for participating in a state-led boycott of Israel will also extend to participation in boycotts led by international governmental organizations such as the United Nations and the European Union.

Jay Michaelson, a legal affairs analyst for The Daily Beast, published a piece on Friday that reached a similar conclusion: The new bill only technically expands the 1977 anti-boycott law, in a way that would make it illegal for U.S. companies to cooperate with any boycotts of Israel and the settlements initiated by the UN or the EU. Michaelson accused AIPAC, the powerful pro-Israeli lobby which supports the legislation, of exaggerating its importance, thereby provoking a harsh response from the left that he says was not in keeping with the language of the draft law.

Not all of the bill’s critics are convinced. “The language in the bill is confusing and doesn’t clearly state what Cardin and Portman wrote in their letter,” one Democratic staffer told Haaretz, adding that “it wouldn’t surprise me if a large number of Democrats will ask to amend this, making it much more clear that citizens expressing support for boycotts will not be punished for their political opinion.” Organizations supporting the BDS movement are worried that even if the bill doesn’t in fact include harsh penalties for supporting boycotts, it could be used to deter and frighten activists who support such boycotts from expressing their opinion.

Democratic members of Congress face a difficult decision on the bill, since many of them have pro-Israeli donors and constituents who would likely be pleased by the passage of a bill making it harder to boycott the country, but they also need the support of the progressive wing of the party, which is paying more and more attention to this legislation in recent days. A key Democratic politician to follow with regards to this bill is Sen. Kirsten Gillibrand of New York, the only Democratic co-sponsor of the bill who is considered a possible contender for the 2020 presidential election. Gillibrand was one of 43 senators who co-sponsored the legislation.

An aide to Senator Gillibrand said: "We have a different read of the specific bill language, however, due to the ACLU's concerns, the Senator has extended an invitation to them to meet with her and discuss their concerns."