Senators in both parties say the odds of a September government shutdown are climbing after President Trump warned that Republicans must scrap the Senate filibuster or face another funding lapse, an ultimatum that caught GOP leaders flat-footed and hardened Democratic resistance a week after the Senate failed to advance the annual defense authorization bill.

Trump linked the filibuster fight to his top legislative priority, the Safeguard American Voter Eligibility Act, which would require documentary proof of citizenship to register, photo identification at the polls, and new curbs on mail-in ballots.

"It's so important that Republicans have to do it. It's so insane. Otherwise, we're going to have a shutdown in September," he said during an appearance on "Fox & Friends" Monday.

Senate Majority Leader John Thune, R-S.D., was blindsided.

"Shut down the government if we don't abolish the filibuster? A new line," he told The Hill, whose reporting first detailed the whip-count anxiety inside the GOP conference. Republicans hold 53 seats, and Thune has repeatedly said the votes to invoke the "nuclear option" and eliminate the legislative filibuster by simple majority are not there.

Senate Democratic Leader Chuck Schumer, D-N.Y., seized on the remarks in a floor speech Monday, arguing the president was telegraphing another funding lapse.

"The Trump shutdown, he's bragging about it. He's predicting it," Schumer said.

He accused Republicans in a "Dear Colleague" letter of jamming through "lopsided" bills that lard up defense while starving nondefense programs.

The posture stiffened Tuesday when Democrats denied cloture on the motion to proceed to the $1.15 trillion FY2027 NDAA, 50-46, a party-line vote in which Thune switched to no to preserve a motion to reconsider.

Senate Armed Services Chair Roger Wicker, R-Miss., called the blockade "unprecedented" and "a new low." Democrats cited the ongoing military conflict with Iran, launched Feb. 28 without congressional authorization, as well as the funding topline.

Sen. Cory Booker, D-N.J., said Trump "wants to shut down this government."

A Democrat senator granted anonymity told The Hill both sides are "testing the waters" over blame ahead of the Nov. 3 midterms, adding, "We don't think Thune has any interest in a shutdown."

Sen. Mike Rounds, R-S.D., said rank-and-file Democrats privately want to avoid a lapse but that leadership "has a different point of view."

Republicans are meanwhile advancing a fallback.

The Shutdown Fairness Act, sponsored by Sens. Ron Johnson, R-Wis., and Todd Young, R-Ind., would guarantee pay for excepted federal employees during a lapse in appropriations.

Sen. Rand Paul, R-Ky., called it "the one good idea that's come out of all this."

Moving the measure would likely require reconciliation and a favorable ruling from the Senate parliamentarian to clear a Democratic filibuster with a simple majority. Appropriations Chair Susan Collins, R-Maine, urged both sides to step back, calling a shutdown "an ultimate failure to govern."

Government funding expires Sept. 30.