Republican presidential candidate Donald Trump slammed rival Hillary Clinton and vowed to abolish "gun-free zones" at the NRA Leadership Forum in Louisville May 20. Here's are key moments from that speech. (The Washington Post)

Donald Trump — who just a few years ago praised President Obama’s appeal for stronger gun control after the mass shooting in Newtown, Conn. — was endorsed and embraced by the National Rifle Association on Friday, completing his rapid transformation into a fierce pro-gun advocate.

Instead of detailing his own positions on gun rights issues at a political forum attended by thousands of NRA members, Trump told the crowd that Democratic front-runner Hillary Clinton would “abolish the Second Amendment” and then release violent criminals from prison, not caring that innocent citizens would be unable to protect themselves.

“Hillary wants to disarm vulnerable Americans in high-crime neighborhoods,” Trump said. “Whether it’s a young single mom in Florida or a grandmother in Ohio, Hillary wants them to be defenseless, wants to take away any chance they have of survival. . . . And that’s why we’re going to call her ‘Heartless Hillary.’ ”

Clinton fired back on Twitter: “You’re wrong, @realDonald­Trump. We can uphold Second Amendment rights while preventing senseless gun violence.”

Maya Harris, Clinton’s senior policy adviser, said Trump’s remarks are “conspiracy theories” that “distract from his radical and dangerous ideas.”

Republican presidential candidate Donald Trump is introduced with Chris Cox (L), Executive Director of the NRA Institute for Legislative Action, and Wayne LaPierre (R), Executive Vice President of the National Rifle Association, at the NRA-ILA Leadership Forum in Louisville on Friday. (Scott Olson/Getty Images)

Gun control has become a dominant issue in the presidential race, though neither party has yet to formally install its nominee. This week highlighted the stark differences between Trump and Clinton, who will be the featured speaker at a Saturday fundraising dinner for a memorial foundation honoring Trayvon Martin, the unarmed Florida teenager shot and killed by a neighborhood watch volunteer in 2012.

Clinton, who has criticized Democratic challenger Bernie Sanders for not being strong enough on gun control, plans to expand that focus in the general election. She has criticized Trump for being cavalier about gun safety and warned in a Twitter message last week that Trump would force schools to allow firearms in classrooms because he no longer wants schools to be gun-free zones.

Clinton has never called for the repeal of the Second Amendment, and fact-checkers have discredited Trump’s assertion that she would abolish it. Instead, she has said she would narrow the loophole that allows easy purchases of firearms at gun shows or online. She also has pledged to seek legislation to end immunity from many lawsuits for gunmakers.

“The gun lobby is the most powerful lobby in Washington,” she said at a recent appearance in Hartford, Conn. “They have figured out how to really intimidate elected officials, at all levels, who basically stop thinking about this problem because they are too scared of the NRA.”

NRA leaders repeatedly warned their members Friday that if Clinton is elected, their individual freedoms will disappear and the Supreme Court will start limiting the rights of gun owners.

“You can kiss your guns goodbye,” said NRA chief executive Wayne LaPierre. “That’s not the America we inherited. That’s not the America we want for our children.”

Anytime a speaker attacked Clinton, the crowd cheered — and Trump kept the attacks going in his partially scripted remarks.

An image of Democratic presidential candidate Hillary Clinton is seen on a screen during the National Rifle Association's NRA-ILA Leadership Forum on Friday in Louisville. (John Sommers II/Reuters)

Trump spoke at this same forum last year, when he was still debating whether he would run for president, and he barely talked about guns beyond declaring his love for the Second Amendment. His lack of specifics then worried some gun rights activists, especially given his history: In 2000, Trump supported a ban on assault weapons and a “slightly longer waiting period” to purchase firearms.

“The Republicans walk the NRA line and refuse even limited restrictions,” Trump wrote in his 2000 book, “The America We Deserve.”

In 2012, after the mass killing at an elementary school in Newtown, Trump praised Obama’s emotional call for policy changes to prevent future shootings.

“President Obama spoke for me and every American in his remarks in #Newtown Connecticut,” Trump tweeted on Dec. 17 of that year.

Trump seemed to accidentally slide into his old ways during his speech Friday while describing the devotion of his two adult sons, Donald Trump Jr. and Eric Trump, to hunting and shooting.

“My sons have been members of the NRA for many, many years, and they’re incredible — they have so many rifles and so many guns, sometimes even I get a little bit concerned,” Trump said to gentle laughter. “I say: ‘That’s a lot.’ ”

Last summer, Trump quietly posted a gun policy paper on his campaign website that called for improving background-check systems, locking up gang members and drug dealers found guilty of gun-related crimes for longer than usual, and fixing the mental health system, although there were no details on how he would do that. He came out against firearm or ammunition bans of any sort, including bans on assault rifles. He also wants to allow concealed-carry permits obtained in one state to be recognized in all 50 states and to get rid of gun-free zones at schools and military bases.

Trump frequently talks about eliminating gun-free zones at his campaign rallies, alleging that they attract “sickos” who want to kill as many people as possible. He said that if the victims had been armed during recent terrorist shootings in Paris and San Bernardino, Calif., “you would have had bullets going in the opposite direction, and believe me, the carnage would not have been the same.”

Trump has a concealed-carry permit in New York, but it is not clear how much experience he has with guns. When an NBC reporter asked him last summer if he ever used the weapon or went to gun ranges, Trump snapped: “That’s none of your business. That’s really none of your business.”

In his NRA address, Trump painted America as a violent and dangerous place where citizens need to arm themselves and always be ready to fight off violent criminals or terrorists. “This is the most basic human right of all, but Hillary wants to strip it away and strip it away from women — and all others,” Trump said.

He accused Clinton of wanting to release drug dealers, gang members and people convicted of violent gun crimes, and said many violent criminals are also illegal immigrants. Earlier in the afternoon, LaPierre accused Democrats of releasing criminals so they could enroll them to vote.

“Heartless hypocrites like the Clintons . . . want to get rid of guns, and yet they have bodyguards that have guns,” Trump said, calling on the Clintons’ bodyguards to “immediately disarm.”

“And let’s see how good they do,” Trump said. “Let’s see how they feel walking around without their guns and their bodyguards. In the meantime, nobody else can have the guns.”

Anne Gearan in Washington contributed to this report.