By Steve Holland

WILMINGTON, N.C. (Reuters) - Republican presidential nominee Donald Trump suggested on Tuesday that gun rights activists could act to stop Democratic rival Hillary Clinton from nominating liberal U.S. Supreme Court justices, igniting yet another fire storm of criticism just as he sought to steer clear of controversy.

"If she gets to pick her judges, nothing you can do folks," Trump said at a rally. "Although the Second Amendment people, maybe there is, I don’t know," he continued.

The U.S. Constitution's Second Amendment guarantees the right to bear firearms.

Before the remark, Trump had been emphasizing his case against Clinton, who is leading in national opinion polls in the race for the Nov. 8 election. Some in the audience in North Carolina who were seated behind Trump could be seen wincing when he made the comment.

Clinton's campaign called the remark "dangerous."

"A person seeking to be the president of the United States should not suggest violence in any way,” it said.  

When asked to clarify what Trump meant, his campaign said he was referring to getting supporters of the Second Amendment to rally votes for Trump in the election.

"It’s called the power of unification – 2nd Amendment people have amazing spirit and are tremendously unified, which gives them great political power," the Trump campaign said in its statement.

Immediately after Trump made his comment, many on social media accused him of effectively calling for Clinton’s assassination. In just three hours, 2nd amendment became the top trending topic on Twitter, with more than 60,000 posts mentioning the term.

Introducing Trump later at another rally in North Carolina, in Fayetteville, former New York Mayor Rudy Giuliani accused the news media of taking the remark out of context to help Clinton get elected.

"What he meant by that was you have the power to vote against her," Giuliani said to cheers. "You have the power to speak against her. You know why? Because you're Americans."

"It proves that most of the press is in the tank for Hillary Clinton," he added. "They are doing everything they can to destroy Donald Trump."

The U.S. Secret Service, which provides security details for both Trump and Clinton and rarely comments on political matters, when asked for a response on Trump, said: “The Secret Service is aware of the comment."

Trump later told Fox News Channel's "Hannity" program that “nobody in that room" thought he meant anything other than to rally support against Clinton.

"This is a strong powerful movement, the Second Amendment," Trump said. "Hillary wants to take your guns away. She wants to leave you unprotected in your home. This is a tremendous political movement."

By day's end, Trump was drawing criticism on several fronts, another chapter in a campaign marked by bitterness and partisanship.

Michael Hayden, a former CIA director who on Monday was among 50 Republican national security experts to denounce Trump in a letter, said on CNN, "You’re not just responsible for what you say. You are responsible for what people hear.”

U.S. Senator Elizabeth Warren, a liberal firebrand who loves tweaking Trump, tweeted that the Republican nominee "makes death threats because he's a pathetic coward who can’t handle the fact that he’s losing to a girl."

Trump senior adviser Kellyanne Conway fought back in a tweet of her own, calling Warren a "disgrace."

GUN RIGHTS AN ISSUE

Gun rights, which have long stirred strong emotions in America, have been a particularly potent issue in the 2016 presidential campaign as violence has convulsed some U.S. cities.

Trump has planted himself firmly on the side of gun owners with a "law and order" campaign. Before his remark about Clinton on Tuesday, he had said Islamic State militants who killed 130 people in France last year could have been stopped if some of the victims had been armed.

The Clinton campaign has challenged Trump when in the past he has accused her of planning to abolish the Second Amendment if elected president. Clinton, in her acceptance speech at the Democratic National Convention, said, "I'm not here to repeal the Second Amendment," saying she wanted "common-sense reforms" to gun laws.

Tuesday's speech came on the heels of a discordant week on the campaign trail for Trump, a businessman seeking his first public office. He came under fire from within his party for belatedly endorsing fellow Republicans in re-election races and a prolonged clash with the parents of fallen Muslim American Army captain Humayun Khan.

On Monday, Trump seemed to be heeding Republican advice to stick to a message of criticizing Clinton and other Democrats while putting forward economic policy proposals in a speech in Detroit.

Trump's vice presidential running mate, Mike Pence, asked if he believed Trump was inciting violence toward Clinton, told NBC’s Philadelphia affiliate: “Of course not. No."

But Democrats called Trump's remarks another sign of a candidate unfit for the White House.

“Don’t treat this as a political misstep. It’s an assassination threat, seriously upping the possibility of a national tragedy & crisis," U.S. Senator Chris Murphy, a Democrat, said in a tweet.

Overall sentiment on social media posts on Trump's remarks was more negative than positive, at a ratio of 2.5 to 1, according to the social media analytics firm Zoomph. #ProtectHillary was also one of the top trending hashtags on Twitter.

The 50 prominent national security officials said in their letter on Monday that Trump would be "the most reckless president in American history."

"He appears to lack basic knowledge about and belief in the U.S. Constitution, U.S. laws and U.S. institutions, including religious tolerance, freedom of the press, and an independent judiciary," their statement said.

(Additional reporting and writing by Alana Wise in Washington and Angela Moon in New York; Editing by Howard Goller and Leslie Adler)