Pablo Martinez Monsivais / AP

WASHINGTON — It didn’t happen after a congresswoman was shot in the head at an official event. It didn’t happen after 20 children were gunned down in their elementary school classrooms. It didn’t happen after terrorists fired on a holiday party at an agency that provided services for people with disabilities in Southern California.

Now, after the worst mass shooting in U.S. history, major new gun control legislation is still not likely to pass in Congress.

Senate Republicans are clearly feeling pressure to address the fact that people whose names surface on terrorist watchlists can easily buy guns. But they do not like a bill that Democrats intend to offer next week that would give the Department of Justice the authority to stop people on the watchlist and other terrorism suspects from buying a gun.

A Republican bill would require the government to delay the purchase of a gun for 72 hours by anyone who is a terrorism suspect, or has been the subject of a terrorism investigation within the last five years, while a review takes place.

“If Democrats are actually serious about getting a solution on that issue, they’d join us,” said Sen. Mitch McConnell, R-Ky., the majority leader. Democrats have argued that the measure, which is approved by the National Rifle Association, is too onerous to have any impact.

Even less likely to pass is a measure that would extend background checks on gun buyers — largely because of hotly contested Senate races in a bitter election year. It seems possible that even Republicans who have supported a background check measure in the past will abandon it this time around.

But unlike in December, when both measures were defeated, some Republicans have been more eager to find a compromise with Democrats. So far, those efforts have not produced a bill both sides can support.

“We have to be serious about this,” Sen. Chuck Schumer, D-N.Y., said Thursday. “We have to pass a real measure that actually keeps guns out of terrorists’ hands, not something that just makes it look like we’re addressing the problem.”

In Orlando on Thursday, President Barack Obama made yet another strongly worded public call — his third this week — for tightening gun restrictions, arguing that the latest mass violence should change the debate. During a two-hour private mourning session with families and friends of the victims at an arena, Obama said, “they pleaded that we do more to stop” gun violence.

“Those who defend the easy accessibility of assault weapons should meet these families,” Obama said, calling on senators to “rise to the moment and do the right thing.”

While Donald Trump, the presumptive Republican presidential nominee, said he would like it to be harder for those on a watchlist to get guns, it was unclear this week which measure he was endorsing, though most Republicans and Democrats said they assumed it was the Republican version.

Election-year politics will make a bill to expand background checks an uphill climb.

After the 2012 mass shooting at Sandy Hook Elementary School in Newtown, Connecticut, Sens. Patrick J. Toomey, R-Pa., and Joe Manchin III, D-W.Va., proposed legislation to extend federal background check laws to internet sales and gun shows.

That bill failed 54-46, with five Democrats opposing and four Republicans — Sens. Susan Collins of Maine, Mark S. Kirk of Illinois, John McCain of Arizona and Toomey — voting yes. After the deaths of 14 people in San Bernardino, California, in December, the Senate again voted down the bipartisan bill 48-50. A measure to stop people on terror watchlists was also defeated.

But Democrats have decided to scrap that background check measure in favor of one sponsored by Sen. Christopher S. Murphy, D-Conn., who led a filibuster of an appropriations bill Wednesday night, that would expand background checks to all gun sales except loans and gifts between family members.

Some prominent Democrats have told fellow party members not to work with Toomey, who is in a tough re-election fight, said three Republicans with knowledge of the conversations. Indeed, Toomey and other Republicans could end up even voting against Murphy’s bill. Democrats lost interest in working with Toomey on proposals to close the so-called terror gap in gun laws when he proposed legislation that gun safety groups did not support.

Toomey will always have his old bill to run on and the fact that he is now working with Everytown for Gun Safety — a group affiliated with former Mayor Michael R. Bloomberg of New York — on legislation to prevent terrorists from obtaining guns.

On Thursday, Republicans came out with their own background check measure, which was sponsored by Sen. Charles E. Grassley of Iowa, chairman of the Judiciary Committee, and included a mental health component. Sen. Susan Collins, R-Maine, also plans to offer an amendment to prevent terrorists from getting guns. Votes on all the measures are scheduled for Monday.

Gun politics infect much of what Congress tries to do; bills to advance changes to the nation’s mental health system are held up over the issue, and that is unlikely to stop. But bipartisan solutions will almost certainly have to wait until the election is over.