A speech that was billed as a major foreign policy address instead unfolded as a savage, mocking evisceration of Donald Trump Thursday as the former secretary of state adopted an aggressive new campaign persona designed to repel the unpredictable challenge posed by the presumptive Republican presidential nominee.

In one of the most striking speeches of her political career, Clinton dispensed with the sober diplo-speak that has characterized her previous national security addresses and went straight for the jugular, unleashing a series of biting attacks on Trump.

In the spirit of President Lyndon Johnson's notorious "Daisy" nuclear blast ad targeting Barry Goldwater's temperament in 1964, Clinton warned that Trump should not be let anywhere near the nuclear codes because he could start a war when somebody "got under his very thin skin."

"He's not just unprepared -- he's temperamentally unfit to hold an office that requires knowledge, stability and immense responsibility," Clinton said during the speech in San Diego, California, days before Tuesday's primary in the Golden State effectively concludes the primary season and confirms her as the presumptive Democratic nominee over Vermont Sen. Bernie Sanders.

Trump fired back while speaking at a rally in San Jose, California, Thursday night.

"I watched Hillary today and it was pathetic. It was so sad to watch," Trump said, calling it a "political speech" that had nothing to do with foreign policy.

"It was a pretty pathetic deal," he added.

The speech marked a significant moment in Clinton's campaign, as it was the first real signal of the tactics and attitude she will use to take on Trump and offered a preview of what are likely to be fierce clashes between the rivals at a trio of presidential debates later in the year. It demonstrated the kind of sardonic, unrestrained humor that she often shows in private interactions with friends and reporters but has refrained from displaying in public.

It also appeared to be aimed at Democrats who are spooked by recent polls showing a tight race between Clinton and Trump, and who fear her often-criticized campaigning skills won't keep up with Trump's volatile and highly effective off-the-cuff style.

And when she argued that Trump's lack of knowledge on foreign policy and temperament would put at risk decades of Republican and Democratic foreign policy advances, she appeared to be making a pitch for disgruntled national security conservatives who feel unable to put their trust in the Republican nominee.

Yet the strategy has its risks, as pretty much all of Trump's GOP primary rivals who tried to take on Trump couldn't survive his return fire. The question is whether Clinton will be more effective. She might be helped by not waiting until the last minute like the Republicans did -- seeking to define Trump early in the minds of the general election audience.

She attempted to convince voters that Trump's ideas are a mix of "bizarre rants, personal feuds and outright lies." She lambasted his "bragging" approach to foreign policy based on a string of "nasty tweets" and accused him of harboring a "bizarre" affinity for authoritarian leaders like Russian President Vladimir Putin, the Communist rulers of China and North Korean dictator Kim Jong-Un.

"We cannot put the safety of our children and grandchildren in Donald Trump's hands. We cannot let him roll the dice with America," Clinton said.

At one point, Clinton imagined Trump composing nasty tweets to respond to her speech. And the combative Republican standard-bearer did not disappoint.

"Bad performance by Crooked Hillary Clinton! Reading poorly from the telepromter! She doesn't even look presidential!" the presumptive GOP nominee wrote as her address ended.

In another tweet, Trump added: "Crooked Hillary no longer has credibility - too much failure in office. People will not allow another four years of incompetence!"

But taking a page from Trump's book, Clinton's speech contained a string of zingers meant to ridicule the presumptive presidential GOP nominee and render him an unacceptable choice for president.

"He says he has foreign policy experience because he ran the Miss Universe pageant in Russia," Clinton said. "The stakes in global statecraft are infinitely higher and more complex than in the world of luxury hotels."

She added: "I will leave it to the psychiatrists to explain his attraction to tyrants" before taking aim at Trump's claim that being a global business tycoon equips him with significant global knowledge.

"You know, there's no risk of people losing their lives if you blow up a golf-course deal," she said. "But it doesn't work like that in world affairs."

"He also says, 'I know more about ISIS than the generals do, believe me.' You know what? I don't believe him," Clinton said, drawing cheers from her partisan audience of around 250 people.

Aides said that Clinton never intended the speech to be a formal foreign policy address but rather to deliver a stinging rebuke of Trump. After watching and reading Trump speeches carefully, Clinton gave an outline of what she wanted to say at the beginning of last week, and speechwriters Megan Rooney and Dan Schwerin worked with foreign policy advisers Jake Sullivan and Laura Rosenberger on the first draft, two campaign aides told CNN.

For the last few days, they've been going back and forth with Clinton to streamline it. Originally, there was more of her own foreign policy, but it was sharpened over last week to include far more Trump.

She spent her coast-to-coast plane ride Wednesday revising the speech with Rooney. Clinton kept working on the specific language right up right before the speech, and a story about Navy SEALs protecting civilians during the 2011 operation that killed Osama bin Laden was a final addition.

In terms of policy, much of what she talked about -- including her views on NATO, trade and Russia -- has already been rolled out in previous, more conventional foreign policy speeches.

But she did focus on the more controversial aspects of the foreign policy that he has laid out.

"This is a man who said that more countries should have nuclear weapons, including Saudi Arabia. This is someone who has threatened to abandon our allies in NATO, the countries that work with us to root out terrorists abroad before they strike us at home," Clinton said.

"He believes we can treat the U.S. economy like one of his casinos and default on our debts to the rest of the world, which would cause an economic catastrophe far worse than anything we experienced in 2008," she continued. "He has said that he would order our military to carry out torture."

Clinton's speech laid out the parameters for what is likely to be a furious foreign policy debate in the general election. It is a feud that will allow her to take aim at Trump's alleged inexperience and lack of knowledge but will also require her to defend what Republicans see as deep vulnerabilities in her own foreign policy record.

Those liabilities include the aftermath of the NATO intervention in Libya, in which she played a dominant role and which left behind a dangerous failed state and a terror haven, as well as the Obama administration's "reset" of relations with Russia, which critics say was naïve and ineffective.

Clinton must also confront accusations that she negligently put American national security at risk by using a personal email server for her official business when she served as secretary of state.

Trump has already made his own attempt to obliterate Clinton's foreign policy credentials.

"She doesn't have the temperament to be president. She's got bad judgment. She's got horribly bad judgment," Trump said last week. "If you look at the war in Iraq, if you look at what she did with Libya, which was a total catastrophe."

The likely Democratic nominee closed out her speech with a preview of how she will respond to such attacks -- by turning the heat back on Trump and his perceived lack of qualifications to be president.

"Imagine Donald Trump sitting in the Situation Room, making life-or-death decisions on behalf of the United States. 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," she said.

"Do we want him making those calls -- someone thin-skinned and quick to anger, who lashes out at the smallest criticism?" she asked rhetorically. "Do we want his finger anywhere near the button?"