WASHINGTON (JTA) — Sen. Marco Rubio, a Florida Republican, said a “significant” number of Democrats in the chamber support the boycott Israel movement. One Democrat, Sen. Chris Murphy of Connecticut, challenged him to put up or shut up.

“I hope a staffer wrote this tweet and when you see it you take it down,” Murphy chided Rubio on Twitter on Monday. “You know it isn’t true that ‘a significant [number] of Senate Dems support BDS’. Really dangerous to play politics with support for Israel.”

BDS refers to the Boycott, Divestment and Sanctions movement targeting Israel.

Rubio had alleged earlier that Democrats were lying about why they oppose legislation he initiated that would, among other things, include a measure meant to fight the BDS movement.

“The shutdown is not the reason Senate Democrats don’t want to move to Middle East Security Bill,” Rubio said on Twitter. “A huge argument broke out at Senate Dem meeting last week over BDS. A significant [number] of Senate Democrats now support #BDS & Dem leaders want to avoid a floor vote that reveals that.”

Rubio’s bill, which the Republican Senate leadership has lined up as the first it will consider this Congress, consolidates four Middle East policy bills that languished in the last Congress, including one that would give states legal cover for penalizing contractors that boycott Israel.

Democrats oppose the bill because they say the priority should be reopening the government. The government has been shut down for more than two weeks by President Donald Trump’s refusal to advance spending bills that don’t include more than $5 billion for a border wall he wants with Mexico.

They also oppose the BDS component because they say legislation inhibiting boycotts infringes on speech rights. Senate Democrats are otherwise on the record as opposing BDS as a movement, and it’s not clear what meeting Rubio referred to, what senators he referred to and what he means by “supporting” BDS.

The argument started when Sen. Bernie Sanders, I-Vt., who caucuses with the Democrats, said Sunday on Twitter that Rubio’s bill was “absurd” because reopening the government should be the priority and its BDS component was unconstitutional.

Republicans have a majority of 53 in the Senate but need 60 votes to advance a bill. Democrats have said they will block Rubio’s bill from advancing,

JTA asked Rubio on Twitter to source his claim. He did not reply, nor did he follow up to Murphy. A staffer for a leading Senate Democrat told JTA that Rubio’s claim was meritless.

Rubio in a separate tweet said Sanders was lying about the constitutionality of his anti-BDS bill.

“My bill doesn’t punish any political activity,” Rubio said. “It protects the right of local & state governments that decide to no longer do business with those who boycott #Israel. So boycotting #Israel is a constitutional right, but boycotting those participating in #BDS isn’t?”

The American Civil Liberties Union, which is leading advocacy against anti-BDS legislation, replied: “First Amendment rights belong to the people, @MarcoRubio — not the government.”

A spokeswoman for Sen. Ben Cardin, D-Md., who in the last Congress introduced his own anti-BDS bill, said he also opposed Rubio’s bill as long as the government is closed and would oppose its advancement, or “cloture.”

“Sen. Cardin has been a long-time champion of federal workers and is opposing cloture on S1 — encouraging others to do so too — because he believes the federal government should be reopened before the Senate proceeds to any other business,” Sue Walitsky told JTA in an email.

Rubio in the meantime launched a separate battle on Twitter with Rep. Rashida Tlaib, D-Mich., who said in a tweet that the bill’s authors “forgot what country they represent.” Rubio said Tlaib was trading in anti-Semitic tropes of dual loyalty.

Tlaib responded that her earlier tweet “was critical of U.S. Senators like yourself, who are seeking to strip Americans of their Constitutional right to free speech.” Rubio is not Jewish.