By Susan Cornwell and Richard Cowan
WASHINGTON (Reuters) - U.S. Democratic Senator Chris Murphy ended a blockade of the Senate after nearly 15 hours on Thursday, saying Republicans had pledged to hold votes on gun control measures to expand background checks and prevent people on U.S. terrorism watch lists from buying guns.
The push in Congress for gun control legislation, which would be the first in the United States in more than 20 years, follows Sunday's massacre at a gay nightclub in Orlando, Florida when a gunman killed 49 people in the worst mass shooting in modern U.S. history.
Republican presidential candidate Donald Trump has also joined the gun debate, announcing on Wednesday he would meet with the National Rifle Association, the country's most powerful gun lobby, to talk about barring people who are on terrorism watch lists from buying guns.
No formal deal between the parties for Senate votes was announced on Thursday, and it was unclear exactly when and how the Senate would proceed with the votes, which would be amendments to an appropriations bill funding the commerce and justice departments.
"We'll try again today to move forward with amendments from both sides and once there is an agreement to do so we'll update everyone," Senate Majority Leader Mitch McConnell, a Republican, said on the Senate floor. But he chastised Democrats for their 15-hour floor speeches on the subject, calling it a "campaign talk-a-thon".
Pressing for gun control votes, Democrats stalled Senate proceedings with all-day speeches on Wednesday and spoke on the Senate floor into the early hours of Thursday.
Republicans, who currently have a 54-person majority in the 100-seat Senate, have blocked a number of Democratic-backed gun control measures over the years, saying they infringe on Americans' constitutional right to bear arms. Some Republican gun control measures - which Democrats have derided as toothless - have also failed to pass.
A senator from Connecticut, Murphy made an impassioned plea for action, saying his own strong desire for change stemmed from the slaughter of elementary schoolchildren at Sandy Hook in his state in December 2012.
"When we began there was no commitment, no plan to debate these measures," he said during the 15th hour of the filibuster early on Thursday.
Murphy said Democrats were given a commitment by the Senate's Republican leadership that votes would be allowed on two Democratic-backed measures on preventing gun sales to people on terrorism watch lists and expanding background checks on would-be purchasers to include gun shows and online sales.
"No guarantee that those amendments pass but we'll have some time to ... prevail upon members to take these measures and turn them into law," Murphy said.
He told CNN the commitment was to schedule the votes for this week or next week.
A Senate Democratic aide said there will likely be four votes - two on Democratic proposals and two on Republican proposals. But even if votes on the proposals are now scheduled, it is not clear whether any of them can gain enough support to pass the Senate.
PAST FAILED EFFORTS
A similar scenario played out on the Senate floor in December following a mass shooting in San Bernardino, California that killed 14 people. Then, competing amendments by Democratic Senator Dianne Feinstein and Republican John Cornyn on curbing weapons sales to people on terrorism watch lists failed.
Murphy said on Thursday that Feinstein had modified her proposal to address many of the Republicans' concern and that she and Cornyn may work out a deal.
Cornyn, the No. 2 Senate Republican, said on Wednesday that he and Feinstein were holding private talks on a possible compromise bill stopping weapons sales to those on the government's terrorism watch lists.
Omar Mateen, the gunman in the shooting in Orlando, had at one point been on such a list but was taken off before the shooting.
Gun control has long been a bitterly divisive topic in the United States. The last time Congress passed a major gun control measure was in 1994, when it passed a ban on semi-automatic assault weapons, such as the one used in Orlando. That expired 10 years later.
President Barack Obama, who has expressed frustration over the failure to pass gun control measures following a string of mass shootings during his time in office, has said the shooting in Orlando was "a further reminder of how easy it is for someone to get their hands on a weapon that lets them shoot people in a school, or in a house of worship, or a movie theater, or in a nightclub." He was set to visit relatives of the dead and survivors in Orlando on Thursday.
Lawmakers and presidential candidates are under pressure to respond to the Orlando attack.
Republican Senator Lindsey Graham, who is not supporting Trump and has been extremely critical of some of his positions, nonetheless applauded his comment that he would meet with the NRA to discuss gun restrictions.
"I want to applaud Mr. Trump. I think he has gone to the middle ground on something that's very important," Graham said on Fox News.
Some Democrats have been skeptical that the outcome of the current push will be any different than before in the Senate.
Late on Wednesday Senate Democratic leader Harry Reid said negotiations "were little more than a smokescreen by Republicans trying to give themselves political cover while they continue to march in lock-step with the NRA's extreme positions."
(Editing by Bill Trott and Frances Kerry)