Democratic presidential hopeful Hillary Clinton has mocked her likely Republican opponent, Donald Trump, citing a lack of foreign policy knowledge. As she was speaking, key Republican Paul Ryan came out to endorse Trump.
At the speech in San Diego, California, Clinton said the presumptive Republican nominee was "peddling foreign policy proposals that are dangerously incoherent."
Deploying a mocking tone at times, Clinton accused Trump of being thin-skinned and unprepared, seeking to juxtapose her own experience with that of her likely opponent in the November 8 election.
"This isn't reality television - this is actual reality," she warned, to applause.
"He is temperamentally unfit to hold an office that requires knowledge, stability and immense responsibility," said Clinton during the speech. "This is not someone who should ever have the nuclear codes."
The speech comes as Clinton and Trump face low ratings. A recent Reuters/Ipsos poll showed half of Trump supporters cited "I don't want Hillary Clinton to win" as the main reason to vote for him, while 41 percent of Clinton supporters say their primary reason is not wanting a Trump presidency.
Foreign policy attacks
Clinton, who served as secretary of state under President Barack Obama, focused on Trump's remarks about US commitment to the NATO alliance, the proliferation of nuclear weapons and the use of torture. She said they demonstrated a lack of knowledge about the US and the world.
"Russia and China lack the strength of US alliances and would love to see them weakened under a potential Trump presidency," Clinton said. "If Donald Trump wins, they'll be celebrating in the Kremlin. We cannot let that happen," she added.
In a foreign policy speech in April, Trump gave few specifics of his foreign policy, but said the US had lacked a coherent foreign policy since the end of the Cold War and that "America first" would be the main theme of his administration.
Clinton said electing Trump president would be a "historic mistake" for the US, portraying him as thin-skinned, irrational and unprepared to be commander in chief. His vision for America is "all wrong," she said, urging voters to remember that the US is a "big-hearted, fair-minded country."
She went on to say that Trump's call to temporarily bar Muslims from entering the US would be a "huge propaganda victory" for the self-proclaimed "Islamic State."
Defending her own record, Clinton mentioned the nuclear accord with Iran championed by President Barack Obama. Trump had previously criticized the diplomatic agreement aimed at dismantling Iran's nuclear program in exchange for easing global economic sanctions.
Clinton challenged Trump to say how he would deal with Iran in the absence of a nuclear deal.
Lack of 'understanding'
Clinton said the next president must bolster America's security by investing at home and sticking together with long-time allies.
Trump has raised the prospect of the US leaving NATO and has said allies must contribute more financially to security agreements with the US.
Clinton said she is offering a "smart and principled" foreign policy that centers on keeping America in a leading role around the globe. She contrasted that with Trump's vision of a "fearful America" that she said would be less secure and less engaged in the world.
Trump, she said, "doesn't understand America or the world."
Clinton also attacked Trump for being too friendly with North Korea and too harsh on European allies. Trump has said he would sit down with North Korean leader Kim Jong Un to try to stop Pyongyang's nuclear program.
Trump hit back via Twitter - as Clinton had joked in her speech he would - to attack Clinton's credibility.
His criticisms to date have centered on Clinton's handling of foreign policy during her 2009 to 2013 stint as secretary of state, including the September 11, 2012, attack by Islamist militants on a US diplomatic facility in Benghazi, Libya, that killed the US ambassador and three other Americans.
He has also cited Clinton's support for the Iraq war, launched by former Republican President George W. Bush, as an example of her hypocrisy.
Ryan endorses Trump
Meanwhile, Paul Ryan, the top elected US Republican, chose the same moment to endorse Trump in the election. The move is seen as a step toward unifying party loyalists behind Trump, with the party split in recent months over its stance on Trump's campaign.
Ryan, who was the 2012 vice presidential running mate of Mitt Romney, has been sharply critical of Trump. "The reality is, on the issues that make up our agenda, we have more common ground than disagreement."