Hillary Clinton's evisceration of Donald Trump

There's a new Hillary Clinton in town.

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.