WASHINGTON — An anti-BDS bill with strong bipartisan backing would not infringe on First Amendment protections, its sponsors said in a letter to the American Civil Liberties Union, which opposes the proposal.

“We cannot state this strongly enough: the bill does not ‘punish US persons based solely on expressed political beliefs,’” says the letter sent Thursday by Sens. Ben Cardin, D-Md., and Rob Portman, R-Ohio.

The measure would expand existing law that bans boycotts imposed by foreign governments to include those imposed by international organizations.

It comes in response to the Boycott, Divestment and Sanctions movement targeting Israel, but also includes the boycott of settlement goods. It was prompted specifically by a decision by the UN Human Rights Council to compile a list of settlement goods and a European Union decision to label settlement goods as being imported from the settlements and not Israel.

“Nothing in the bill restricts constitutionally protected free speech or limits criticism of Israel and its policies,” the letter says. “Instead it is narrowly targeted at commercial activity and is based on current law that has been constitutionally upheld.”

Companies could still boycott Israel, the letter says, and not face repercussions. However, cooperating with an international organization’s boycott — for instance, providing information to the UN Human Rights Council on an American company’s dealings with an Israeli company that operates in the settlements — would incur penalties.

The ACLU, in a letter to senators urging them not to back the measure, said that “the bill would punish businesses and individuals based solely on their point of view. Such a penalty is in direct violation of the First Amendment.”