Republican White House hopeful Donald Trump signalled that he could support banning people on terror watch lists from purchasing guns, a move that would place him in opposition to members of his own party.

Trump, the presumptive Republican presidential nominee, regularly touts his support for the constitutionally enshrined right to bear arms.

He has said after terrorist attacks, including the mass shooting in Orlando, Florida on Sunday, that death tolls would have been lower if private citizens had been armed and able to shoot back.

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But he suggested on Wednesday that he is prepared to consider restrictions on gun purchases after it was revealed the Orlando shooter legally bought a rifle and handgun in Florida despite having been investigated by the FBI about possible extremist ties.

Trump's announcement may have placed him on a collision course with the National Rifle Association (NRA), which tweeted on Tuesday that "restrictions like bans on gun purchases by people on 'watch lists' are ineffective, unconstitutional, or both."

But instead, the group opened the door to the Republican flagbearer, saying they would be "happy to meet" with Trump.

The NRA, a politically influential lobbying group that claims more than four million members and has played a key role in thwarting gun control legislation in the US Congress, endorsed Trump on May 20. 

According to Senate Democrat Bill Nelson of Florida, the Federal Bureau of Investigation (FBI) said Orlando nightclub gunman Omar Mateen was placed on a terrorism watch list from 2013 until 2014.

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Republican lawmakers and the NRA previously refused to support legislation that would deny weapons to people on such lists, arguing that such a bill would infringe on the Second Amendment rights of everyday Americans, including those who may have been placed unfairly on watch lists or no-fly lists.

"Many Republicans can't accept a situation where the president of the US is deciding, without any oversight of the courts, who goes on the no-fly list and therefore restricting the access of many Americans to buying guns," said Al Jazeera's Alan Fisher, reporting from Washington DC.

But Trump may be feeling "cornered" and may be willing to compromise with the Democrats, Fisher said.

"Clearly, Trump has been concerned by some of the publicity over the past 48 hours and concerned as well that the Democrats are going to push very hard on the fact that if you are on the no-fly, there should be a no buy when it comes to guns."

Frustrated Democrats took to the Senate floor on Wednesday to launch a filibuster - a procedural obstruction - and call for action to restrict suspected terrorists' access to guns.

'No fly - No buy'

"What we are going to hear a lot about over the next few days, is something called 'no fly-no buy'," Fisher said.

"It means if you are in a no-fly list, you shouldn't be allowed to buy a weapon."

"It's something that Barack Obama has said on several occasions."

A US government report shows that known or suspected terrorists have passed background checks for gun sales more than 90 percent of the time under the current legislation.

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Source: Al Jazeera and agencies