The American Civil Liberties Union called on members of Senate on Wednesday to reject proposed legislation that would make it a felony for U.S. citizens to support boycotts of Israel and its West Bank settlements.

The ACLU warned that the legislation, which is sponsored by a senior Democratic Sen. Ben Cardin of Maryland, seeks to "impose civil and criminal punishment on individuals solely because of their political beliefs about Israel and its policies."

Cardin's proposed bill, dubbed by him as "the Israel Anti-Boycott Act," is supported by AIPAC, the powerful pro-Israeli lobby group, which has taken pride in promoting this legislation in recent months. One of the main reasons the ACLU has come out strongly against it is that violations of the bill could lead to a minimum civil penalty of $250,000 and a maximum one of $1 million, or a lengthy period in jail.

In its letter to all the members of the Senate, the ACLU – one of the oldest and most prominent civil rights organizations in the United States – wrote that "this bill cannot fairly be characterized as an anti-discrimination bill," and added that while the organization will take no position regarding the boycott question, "such boycotts, whatever their merits, rightly enjoy First Amendment protection. By penalizing those who support international boycotts of Israel, the bill seeks only to punish the exercise of constitutional rights."

The bill has received support from a number of senior Democratic politicians, including Senate Minority Leader Chuck Schumer of New York, and Sen. Richard Blumenthal of Connecticut. A similar bill that was introduced in the House of Representatives has been endorsed by senior Democrats including Steny Hoyer of Maryland, the second-ranking Democratic member of the House, and Adam Schiff of California, the most senior Democrat on the House Intelligence Committee.

The letter published by the ACLU is expected to generate pressure on these Democrats and others who have expressed support for the legislation, to demand changes to the bill's language, especially the part of it referring to criminal punishments for the violators. Already on Wednesday, a number of Democrats who were contacted by the website The Intercept admitted that they were not aware of the exact content of the legislation and would examine it closely in light of the ACLU's warnings.