INDIANAPOLIS — President Trump on Friday told gun rights activists that Democrats have never been “angrier” even amid the country’s recent successes, particularly because their claims of collusion between his campaign and Russia have been exposed as false.

“America’s future has never been brighter, and yet Democrats have never been angrier, especially now that their collusion delusion has been exposed to the world as a complete and total fraud,” Mr. Trump said to cheers and applause. “It’s been the greatest political hoax ever in our country.”

Special counsel Robert Mueller’s recent report did not find sufficient evidence to prove that Mr. Trump’s campaign colluded with Russia in that country’s efforts to interfere in the 2016 U.S. presidential election.

Mr. Trump was addressing activists at the annual “leadership forum” hosted by the National Rifle Association’s legislative-lobbying arm.

He called himself a “champion” of the Second Amendment and vowed to defend it.

“It’s under assault — but not when we’re here. Not even close,” he said. “I’ll never let you down.”

The appearance is Mr. Trump’s third consecutive address to the NRA since taking office — underscoring the deep ties between the president and gun rights activists, who will comprise a key part of the GOP base heading into the 2020 election.

“I’m ecstatic about what he’s done in office,” Martha Jacobsen, 62, a programmer/data analyst from Kansas, said as she stood just outside the main area in Lucas Oil Stadium where Mr. Trump and others were speaking. “Regardless of your political views, it’s nice to see the country engaged in debate. No matter which side people are [on], there’s not as much apathy.”

The president did irk some gun rights enthusiasts when his administration imposed a recent ban on bump stock-type devices.

Bump stocks attach to semiautomatic weapons to mimic the rate of machine-gun fire and gained attention after the October 2017 shooting in Las Vegas, where the gunman used them to rain fire down on concertgoers, killing 58 people.

Still, gun rights advocates remain fiercely loyal to Mr. Trump, pointing to his two U.S. Supreme Court picks, his general advocacy of Second Amendment rights since taking office and his willingness to mix it up with Democrats and the media.

They had set their hopes even higher after the 2016 elections that a GOP-controlled Washington, led by Mr. Trump, would take steps not only to block new gun controls, but to expand gun rights at the federal level.

But legislative priorities like expanding concealed carry rights and loosening regulations on gun-sound suppressors, or silencers, stalled out amid recent shootings.

Those proposals are now nonstarters with a new Democratic majority in the House, which has moved in the other direction this year by passing legislation to expand gun-purchase background checks and extend the amount of time a gun dealer has to wait to hear back from the FBI before proceeding with a sale.