By Steve Holland

WASHINGTON (Reuters) - The FBI report scolding Democrat Hillary Clinton over her email practices as secretary of state plays into a chief vulnerability that her rival, Republican Donald Trump, hopes to exploit – wariness among U.S. voters about her trustworthiness.

Clinton campaign spokesman Brian Fallon said the Federal Bureau of Investigation's recommendation against criminal charges for Clinton means the email matter “is now resolved.”

Yet even as he removed the threat of charges against the Democratic candidate, FBI Director James Comey described Clinton's handling of classified information as “extremely careless.”

Trump, the presumptive Republican nominee, seized on that comment.

"We're talking about serious stuff," he told a rally in Raleigh, North Carolina. "The laws are very explicit."

The New York businessman has been struggling to unify the party around his candidacy, and Republicans say Trump could use the issue to heal some of the rifts within his party while motivating conservatives to go to the polls in the Nov. 8 election.

Indeed, a main element of Trump's strategy for winning over Republicans who are lukewarm on his candidacy is to emphasize the importance of keeping Clinton out of the White House.

"Clinton won the 'Comey primary' but Trump has the opportunity to drive home the lack of trust theme," said Republican strategist Scott Reed.

A Reuters/Ipsos poll from May 1-11 found that a majority of American adults do not think Clinton is an honest person. This included 89 percent of Republicans, 72 percent of independents and 36 percent of Democrats.

More than a quarter, 27 percent, of Clinton's own supporters did not think she is an honest person, according to the poll.

Democratic strategist Steve Elmendorf said Clinton needs to move quickly to get past the email episode and talk about issues central to her campaign. He predicted the issue will be short-lived as voters focus on the candidates' vice presidential running mates, the nominating conventions and fall debates.

"She made a mistake here, there's no question about it," said Bob Shrum, a Democratic strategist who was chief strategist for 2004 nominee John Kerry, the current secretary of state.

"But there’s also justice in what she says, that for 24 years the Republicans have relentlessly gone after Bill and Hillary Clinton. That has its impact and that impact you can see in her trust numbers. But this decision, despite how anybody spins, it not only helps her but in my view was indispensable to her," Shrum said.

Hours after Comey's announcement, Trump's campaign sent an email soliciting money based on the Clinton email controversy.

The big question for Republicans is whether their presumptive presidential nominee will be able to prosecute the case effectively against Clinton since he is still at war with fellow Republicans and has little in the way of campaign funds to pay for TV ads against her.

"I don’t think there’s a way that Clinton can use this to raise anger at Republicans," said Andy Smith, director of the University of New Hampshire Survey Center. "But the Republicans can certainly use this to raise anger at Clinton and the Democrats. Anger is a great motivator to get people out to vote."

Trump's initial response showed why Republicans continue to have concerns about him. Interviewed by Fox News' Bill O'Reilly, Trump spent as much time criticizing Comey as he did Clinton. Comey is respected by Republicans and Democrats alike.

"I have such respect for the FBI that I just don’t understand what happened here," Trump said. "Everybody knows she’s extremely guilty. And they really said she was guilty today and then they said we’re not going to prosecute."

Ryan Williams, a Republican strategist and former spokesman for 2012 nominee Mitt Romney, called the FBI report a "disqualifying event" for Clinton.

"Trump's singular focus should be that she acted inappropriately. But Trump has a knack for distracting from Hillary Clinton's scandals with his gaffes and missteps," he said.

Trump has been fighting allegations of anti-Semitism in recent days over his campaign's use of a Jewish star in a tweet attacking Clinton as corrupt.

(Writing by Steve Holland; Editing by Caren Bohan and Leslie Adler)