By Richard Cowan

WASHINGTON (Reuters) - The U.S. Senate on Monday rejected four measures restricting guns after last week's mass shooting in an Orlando nightclub, although lawmakers were still trying to forge a compromise that could keep firearms away from people on terrorism watch lists.

In a familiar setback for gun control advocates, all four of the measures to expand background checks on gun buyers and curb gun sales to those on terrorism watch lists - two put forth by Democrats and two by Republicans - fell short of the 60 votes needed for passage in the 100-member chamber.

The deadliest mass shooting in modern U.S. history last week had intensified pressure on lawmakers and spurred quick action, but the gun-control measures lost in largely party-line votes that showed the lingering political power in Congress of gun rights defenders and the National Rifle Association.

Republicans and their allies in the NRA gun lobby said the Democratic bills were too restrictive and trampled on the constitutional right to bear arms. Democrats attacked the Republicans' plans as too weak.

"It's always the same. After each tragedy, we try, we Democrats try to pass sensible gun safety measures. Sadly, our efforts are blocked by the Republican Congress who take their marching orders from the National Rifle Association," Senate Democratic Leader Harry Reid of Nevada said.

Senate Majority Leader Mitch McConnell, a Kentucky Republican, said the Democratic measures were ineffective and Democrats were not sincere in their effort.

"Instead of using this as an opportunity to push a partisan agenda or craft the next 30-second campaign ad," McConnell said, Republican senators "are pursuing real solutions that can help keep Americans safer from the threat of terrorism."

Gun control efforts failed after mass shootings at an elementary school in Newtown, Connecticut, in 2012 and a conference centre in San Bernardino, California, in 2015. But some senators see resistance to gun restrictions softening as national security looms larger in the debate.

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The Orlando gunman, Omar Mateen, pledged allegiance to the militant group Islamic State as he killed 49 people in a gay nightclub.

"This country is under attack ... it's not a plane or an explosive device, it's an assault weapon," said Murphy, who led a 15-hour filibuster last week to draw attention to the effort to restrict guns.

A Reuters/Ipsos poll conducted last week found that 71 percent of Americans favour at least moderate regulations and restrictions on gun sales. That compared with 60 percent in late 2013 and late 2014.

Senior Senate aides on Monday left open the possibility of other votes later in the week on unspecified gun control proposals. Some Republicans pinned hopes on a compromise proposal by Senator Susan Collins, a Maine Republican, which was not one of the four bills being considered on Monday.

Her plan would restrict gun purchases to a narrow group of suspects, including those on a "no-fly" list or a "selectee" list of people who require additional screening at airports..

Even if the Senate approved a gun compromise, it would also have to be passed by the Republican-majority House of Representatives. House Republican leadership aides did not comment on the possibility that any bills proposing gun restrictions would be considered on the House floor this week.

Congress has not passed new gun restrictions since a 2007 expansion of the government's automatic background check database to include individuals with a history of mental illness and felons. The United States has more than 310 million weapons, about one for every citizen.

(Additional reporting by Amanda Becker and Emily Stephenson; Writing by John Whitesides; Editing by Alistair Bell and Mary Milliken)