After Senate Majority Leader Mitch McConnell (R-Ky.) delayed until Thursday afternoon the second procedural vote on a bill that would punish boycotts of Israel, Palestinian rights groups that have condemned the legislation as "unconstitutional" kept up urgent calls for Americans to pressure their senators to block the measure again.

"You still have time to contact your Senators to again defeat this unconstitutional bill that encourages states to punish people for boycotting for Palestinian rights," declared Josh Ruebner, policy director at the U.S. Campaign for Palestinian Rights.

The procedural vote on the legislation—S.1—is expected to come as early as 1:45 pm ET on Thursday.

If passed, S.1—which is sponsored by Sen. Marco Rubio (R-Fla.)—would hand states and localities more power to punish companies and individuals who participate in the the Boycott, Divestment, and Sanctions (BDS) movement.

The ACLU has denounced the legislation as an unlawful attack on free speech, and two federal courts have ruled that similar bills are unconstitutional.

Condemning backers of the Senate bill as "betrayers of the Constitution" in a blog post on Tuesday, Juan Cole of Informed Comment pointed out that "[e]conomic boycotts have been part and parcel of American political striving for liberty from the beginning. I have three words for you: Boston Tea Party. What do you think the American colonists were doing when they tossed 342 chests of British tea into the harbor? They were boycotting, divesting, and sanctioning the injustice of King George III."

"Two federal judges have already found state laws that attempt to punish companies or individuals for boycotting Israel unconstitutional, one in Kansas and one in Arizona," Cole concluded. "Of course they are unconstitutional. They are also racist, aimed at keeping brown Palestinians down."