Sen. Pat Toomey (R-Pa.) is working with former New York mayor Michael Bloomberg’s gun-control advocacy organization on alternative legislation to close the terror gap, the group said Wednesday.

Efforts to strike a gun control compromise in the wake of the Orlando massacre were fraying by the end of Wednesday, even though Senate Democrats continued their blockade of the floor in a bid to deny guns to suspected terrorists.

Early Wednesday, Sen. Pat Toomey (R-Pa.) was in talks with a gun control advocacy organization backed by former New York mayor Michael Bloomberg to craft a measure that Republicans could live with to prevent terrorists from obtaining firearms.

“This is not rocket science to figure this out,” Toomey said on the Senate floor Wednesday, adding that he had been speaking to “several” senators about a compromise.

But by the day’s end, leading Democrats were deriding the specifics of the Pennsylvania Republican’s proposal, and a spokeswoman for Bloomberg’s organization, Everytown for Gun Safety, said: “We’re not there yet.”

Meanwhile, attempts to get Republicans to support another measure appeared to fall short.

Sen. Dianne Feinstein (D-Calif.) had approached Sen. John Cornyn (R-Texas) to help craft a measure that would allow the attorney general to prevent any terrorist suspects from acquiring guns. Underscoring Republican concerns with due process, Cornyn had previously proposed requiring court approval within three days for a ban on an individual’s attempted purchase of a gun.

Both measures were introduced in 2015 and failed following the San Bernardino, Calif., shooting.

“I don’t think it’s going to work out,” Feinstein told reporters late Wednesday of her efforts to find a middle ground with Cornyn.

How to prevent terrorists from getting guns has become a subject of intense debate in Congress in the wake of the deadly attack in Orlando that killed 49, the worst mass shooting in U.S. history. The shooter, 29-year-old Omar Mateen, had been on the FBI’s terror watch list, but was removed in 2014 for lack of evidence that he would commit an immediate crime.

[Senators target terrorist watch lists]

Democrats want to add the gun control proposals to pending legislation that would fund several agencies, including the Justice Department.

Democratic senators are putting their collective weight behind two gun control measures: Feinstein’s proposal, and a measure jointly written by Sens. Toomey and Joe Manchin (D-WVa.) to require criminal and mental health background checks at gun shows and for online firearms sales.

Manchin called his measure with Toomey, which garnered 54 votes including the support of four Republicans the last time the Senate considered it in 2013, “the fundamental building block” necessary for any effort to keep guns away from terrorists.

“You can’t do a no-fly bill, you can’t do a terrorist watch list, and then leave a loophole,” Manchin told reporters. “That loophole must be closed.”

But by evening, there was no agreement on what amendments would be given a vote – and Democrats, who began their filibuster at 11:21 a.m. on Wednesday, were refusing to yield the floor.

“I am prepared to stand on the Senate floor and talk about the need to prevent gun violence for as long as I can,” Sen. Chris Murphy (D-Conn.), whose state was the site of the Newtown massacre at an elementary school in 2012, tweeted.

Sen. Chris Murphy (D-Conn.) is launching a filibuster and demanding a vote on gun control measures. The move comes three days after 49 people were killed in a mass shooting in Orlando. (AP)

Over 30 other Democrats came to the floor throughout the day to help Murphy in his efforts, which organizers pledged to continue into the night.

Feinstein, the ranking member on the Intelligence Committee, said she was preparing a version of her proposal that would deny guns to those who were once on the terror watch list going back five years. That would be enough to cover Mateen, who was removed from the watch list in 2014.

But she said that when presented with the idea, Toomey told her “no Republican would support it – which indicated to me that the [National Rifle Association] has spoken.”

Toomey proposed his own language on Wednesday in which a secret Foreign Intelligence Surveillance Court would annually vet a list of potential terrorists from the attorney general, who could be prevented from purchasing a firearm or explosive if the court agreed.

A Toomey spokesman said that the Bloomberg-backed organization had promised to back the proposal “so long as we generated bipartisan support.”

But Sen. Charles Schumer (D-N.Y.) was lumping Toomey’s proposal in with Cornyn’s, and calling it “not at all acceptable.”

There were others signs that the tide is shifting among Republicans toward banning those on the terrorist watch list — which included some 800,000 people as of September 2014 — from purchasing firearms.

Presumptive GOP presidential nominee Donald Trump tweeted Wednesday morning that he would be meeting with the NRA to express his views that those on the terrorist list should not be allowed to purchase weapons.

Sen. Susan Collins (R-Maine), who is not up for reelection until 2020, criticized the Cornyn measure — which she supported last year — as insufficiently strong, saying that it “doesn’t do the job” and expressing hope for a more limited version of Feinstein’s measure.

Senate Majority Leader Mitch McConnell (R-Ky.) suggested Tuesday that he is “open” to keeping guns out of the hands of terrorists, but not with the current proposals on the table. “Nobody wants terrorists to have firearms,” McConnell remarked.

[McConnell ‘open’ to keeping guns from terrorists, but not in the way Dems want]

The Cleveland Plain Dealer reported Tuesday that Sen. Rob Portman (R-Ohio), who is facing a difficult reelection this year, was ready to back a federal ban on weapons sales to anyone on a terrorist watch list — though a spokesman for him later said that he would now support a waiting period instead of a blanket ban on a suspected terrorist buying guns.

And Sen. Lindsey Graham (R-S.C.) said Tuesday that he believed the FBI should be alerted when a suspected terrorist goes to buy a gun, but he stopped short of advocating an all-out ban, arguing that anyone who triggers an alert should have a chance to prove why the FBI might be wrong.

Meanwhile, Senate Democrats rolled out another gun control measure that, like the Feinstein legislation, they would like to see attached to the Commerce, Justice and Science appropriations bill.

It would funnel $190 million to counter-terrorism efforts and active-shooter training at the FBI, as well as help the information-sharing efforts at the Bureau’s Terrorist Screening Center. It would also underwrite additional surveillance capabilities, creating 36 new positions, including 18 for special agents.

Senate Minority Leader Harry Reid (D-Nev.) has been reminding lawmakers that U.S.-born al-Qaeda operative Adam Gadahn, who was killed in a 2015 drone strike in Pakistan, advised radical sympathizers in America to go to a gun show and buy firearms in 2011.

“These murderers are calling on their radical followers to exploit loopholes in America’s gun laws. Why? Because firearms are easier to obtain in America than making their homemade bombs,” Reid said.

House Democrats gathered Wednesday morning in a room in the basement of the Capitol to plot a path forward. Afterward, leaders said they hoped to press for more thorough gun controls, including expanded background checks and a renewal of the lapsed assault weapons ban, but said they are focused first on closing “the terror gap.”

“We have a pretty simple message — this Congress should not leave this week without giving us a vote on something very simple: ‘No Fly, No Buy,’” said Rep. Xavier Becerra (D-Calif.), chairman of the House Democratic Caucus. “If you can’t fly because you’re on a watchlist by our government and you’re not allowed on an airplane, you should not be able to enter a gun shop and walk out with an assault weapon.”

Mike DeBonis and Catherine Ho contributed to this report.