US Democratic presidential candidate Hillary Clinton speaks at Transylvania University in Lexington, Kentucky, US, May 16, 2016. [Photo/Agencies]

WASHINGTON - Democratic presidential front-runner Hillary Clinton pivoted to a general election match-up against Republican candidate Donald Trump on Thursday, saying he is dangerously unpredictable and not qualified to be president.

Confident that she is finally close to defeating US Senator Bernie Sanders of Vermont for the Democratic nomination, Clinton turned heavy fire on Trump, who has been running about even with her in national polls of voters looking ahead to the Nov. 8 presidential election.

On the Republican side, Trump promoted top aide Paul Manafort to serve as campaign manager and chief strategist, the Trump campaign said. Corey Lewandowski, the Trump aide who has been campaign manager, retains that title and will continue to oversee day-to-day operations, the campaign said.

In addition, Trump has hired veteran Republican lawyer A.B. Culvahouse to help vet potential vice presidential running mates, a source close to the campaign said.

In a CNN interview, Clinton used the example of the apparent downing of an EgyptAir plane from Paris to Cairo to say that Trump would lack the skills to bring together US allies to respond to global threats.

"I know how hard this job is and I know we need steadiness, as well as strength and smarts in it, and I have concluded that he is not qualified to be president of the United States," Clinton said.

Trump, the Republicans' presumptive presidential nominee, has been intensifying his criticism of Clinton by lobbing personal attacks at her and her husband, former President Bill Clinton.

Clinton, a former US secretary of state, said she would resolutely refuse to respond to Trump's goading. "He can say whatever he wants," she said.

But she said the EgyptAir crash reinforces the need for American leadership and that Trump's proposed temporary ban on Muslims entering the United States has sent the wrong signal to countries that Washington will need to work with in the fight against Islamic militants.

"He says a lot of things that are provocative, that actually make the important task of building this coalition, bringing everybody to the table and defeating terrorism more difficult," she said. "It sends a message of disrespect and it sends a message that makes the situation inside those countries more difficult."

Clinton suggested the Democratic race was over because of her nearly insurmountable lead in delegates to the nominating convention, despite Sanders' insistence on staying in the race.

"I will be the nominee for my party," she said. "That is already done, in effect. There is no way that I won't be."

She said Sanders will have to eventually help her unify the Democratic Party after the prolonged nomination fight.

"I am absolutely committed to doing my part, more than my part. But Senator Sanders has to do his part," she said.

The Sanders campaign rebuffed Clinton's nudge to get out of the race, pointing to his recent victories.

"In the past three weeks voters in Indiana, West Virginia and Oregon respectfully disagreed with Secretary Clinton. We expect voters in the remaining eight contests also will disagree," Sanders spokesman Michael Briggs said.

Trump stepped up efforts to rally Republican loyalists behind his campaign after winning a divisive primary fight that left the party ruptured.

On Capitol Hill, Manafort and other Trump aides met with conservatives in the House of Representatives who are members of the Freedom Caucus group and canvassed them for policy ideas.

"Manafort was reaching out for ideas" on policy, and several Freedom Caucus members made suggestions, said Republican Representative Scott DesJarlais of Tennessee.

"It went very well, it was encouraging. I think the Trump team recognizes the relevance of the Freedom Caucus, and the influence they have. I think actually, despite some early skepticism by some members, I think the (Freedom Caucus) board received Manafort and his representation of Trump very well," DesJarlais said.

Manafort also met with US Senator Mike Lee of Utah, a Tea Party Republican who was a big backer of US Senator Ted Cruz of Texas for president. Lee has expressed some concerns about Trump's candidacy.

A spokesman for Lee said US Senator Jeff Sessions of Alabama, a key Trump adviser, was also at the meeting.