US presidential hopeful Hillary Clinton tore into her likely election rival Donald Trump as never before Thursday, saying his foreign policy is dangerously incoherent and labeling him unfit for office.

"He is temperamentally unfit to hold an office that requires knowledge, stability and immense responsibility," Clinton said, cranking up the rhetoric in what is already a deeply acrimonious election season.

"This is not someone who should ever have the nuclear codes."

Americans will go to the polls on November 8 to elect a new president, replacing Barack Obama who steps down after eight years in office.

Clinton and Trump are all but assured to be the Democratic and Republican party nominees.

Clinton, who formerly served as secretary of state, first lady and US senator, is vying to become the first female president in America's 239-year history.

Trump, speaking at a rally in San Jose, California on Thursday night, ridiculed the Democrat's speech.

"It was pathetic. It was so sad to watch. She was up there, supposed to be a foreign policy speech, it was a political speech, had nothing to do with foreign policy."

Earlier, the businessman took to his favorite social media platform Twitter as Clinton was speaking: "Bad performance by Crooked Hillary Clinton! Reading poorly from the teleprompter! She doesn't even look presidential!"

The 69-year-old celebrity real estate mogul burst onto the political scene last year, leveraging his fame to garner media coverage and dispatching much more experienced Republican rivals.

But in the process Trump has rejected many of the central tenets of foreign policy endorsed by both major parties.

He has questioned the utility of NATO, expressed admiration for Russian President Vladimir Putin and North Korean leader Kim Jong-Un and appeared cavalier about the prospect of nuclear weapons proliferation.

Clinton ridiculed her foe -- painting him as a thin-skinned neophyte -- and voiced incredulity at his views.

"I will leave it to the psychiatrists to explain his affection for tyrants," she said, while trying to showcase her own grasp of world affairs.

"Even if I weren't in this race, I'd be doing everything I could to make sure Donald Trump never becomes president because I believe he will take our country down a truly dangerous path," she told an audience in San Diego, California.

- Brutal fight -

Clinton's fiery speech foreshadows what is certain to be one of the most vitriolic US elections in living memory.

With the two major parties deeply divided after rancorous primary campaigns, both leaders are likely to use a common enemy to unify supporters.

By taking on Trump on national security, Clinton is trying to move the battle to favorable terrain, according to Geoffrey Skelley of the University of Virginia Center for Politics.

"She definitely does best against Trump on issues of security, foreign policy," he said.

"It's something that she thinks is going to be a key driver in the end to her winning, that people are going to decide that -- even if they are uncertain about Clinton -- that she is the safer choice."

On Thursday, Trump got some succor, winning the endorsement of Republican Paul Ryan, speaker of the House of Representatives.

But even as Trump wins the backing of Republicans in Congress, many party acolytes who take pride in Republicans' traditionally strong national security credentials are hostile to Trump leading the party.

Clinton will hope her message resonates with military families and establishment Republicans -- prominent in vital battleground states like Virginia and North Carolina.

Some of her message appeared tailored to them.

"Imagine Donald Trump sitting in the situation room making life or death decisions on behalf of the United States," Clinton said. "Imagine him deciding whether to send your spouses or children into battle."

"Imagine if he had not just his Twitter account at his disposal when he's angry but America's entire arsenal. Do we want him making those calls? Someone thin-skinned and quick to anger who lashes out at the smallest criticism? Do we want his finger anywhere near the button?"