Hillary Clinton, a former US senator, has already delivered several speeches on foreign policy and national security.
Democratic presidential front-runner Hillary Clinton will berate Republican Donald Trump for being too friendly with North Korea and too harsh on European allies in a foreign policy speech on Thursday aimed at portraying the billionaire businessman as unfit for the White House.
The speech in San Diego comes as the former secretary of state seeks to shift her attention to the November 8 presidential election against likely rival Trump and away from Bernie Sanders, the US senator from Vermont who is continuing his longshot bid for the Democratic nomination.
Trump has said he would sit down with North Korean leader Kim Jong Un to try to stop Pyongyang's nuclear programme and has criticized the decades-old NATO alliance with mainly European nations as obsolete and too costly for the United States.
"It's important that people understand this is not just about Republican versus Democrat, that Trump is unlike any presidential candidate we have seen and he is fundamentally unfit to be our commander in chief," Clinton aide Jake Sullivan said in an interview before the speech.
California is among six states holding Democratic nominating contests on Tuesday and Clinton, who holds a nearly insurmountable lead over Sanders in the delegate count, is hoping a decisive win there can help her clinch the nomination.
Clinton, a former US senator, has already delivered several speeches on foreign policy and national security. Thursday's address will not break new policy ground, but will mainly respond to recent comments by Trump, her campaign said.
She will argue that Trump's statement that he would be willing to talk to Kim Jong Un, made in a Reuters interview in May, has only emboldened the North Korean leader, Sullivan said.
A column this week in DPRK Today, one of North Korea's state mouthpieces, described Trump as a "wise politician" and Clinton as "thick-headed Hillary."
"Donald Trump's statements about North Korea show that he has more interest in making Kim Jong Un like him than backing up our friends and allies in the region," Sullivan said, noting that South Korea has worked with the United States on missile defence.
Trump, who has never held elected office, has talked tough on foreign policy. He embraced the use of waterboarding and other brutal interrogation techniques for terrorism suspects and vowed to renegotiate trade deals and ask members of the 28-nation NATO alliance to "pay up" or "get out."
Sullivan, who worked with Clinton at the US State Department, said Clinton will highlight her record of getting allies to "step up" and share defence costs but would not walk away from an alliance she views as important to US efforts on counterterrorism, nuclear proliferation and other issues.
At a rally Wednesday night in Sacramento, California, Trump said he had seen a copy of Clinton's speech and "it was such lies about my foreign policy."
Trump has criticized Clinton for her handling of foreign policy during her 2009-2013 stint as foreign secretary, including the September 11, 2012, attack by Islamist terrorists on a US diplomatic facility in Benghazi, Libya, that killed the US ambassador and three other Americans. He cites Clinton's support for the Iraq war - launched by former Republican President George W. Bush - as another example of her shortcomings.
Using what is now his trademark epithet for Clinton, Trump kept up the assault on Thursday on Twitter, saying "Crooked Hillary Clinton has zero natural talent - she should not be president. Her temperament is bad and her decision making ability-zilch!"
The chairman of the Republican National Committee, Reince Priebus, also weighed in with a statement against Clinton.
"There isn't a more flawed messenger on national security issues than Hillary Clinton, who as Obama's secretary of state helped turn Libya into a jihadist playground, spearheaded the dangerous nuclear deal with Iran, and secretly called for bringing terrorists from Guantanamo onto US soil," he said.
In assailing each other's suitability for the White House, Clinton and Trump are reflecting a negative voter mood ahead of next month's party conventions that will choose the presidential nominees.
Both Clinton and Trump are facing record low favourability ratings. A Reuters/Ipsos poll taken on Friday through Tuesday shows half of Trump supporters say the primary reason they are going to vote for him is "I don't want Hillary Clinton to win," while 41 per cent of Clinton supporters cite their primary reason as not wanting Trump to win.
© Thomson Reuters 2016