A supporter of Republican US presidential candidate Donald Trump sits on the airport tarmac during a campaign rally in Redding, California, U.S. June 3, 2016. [Photo/Agencies]

WASHINGTON - US Democratic front-runner Hilary Clinton has launched a blistering attack against Republican nominee Donald Trump as the war between the two rivals began in an election expected to be a knock-down drag out fight.

Clinton lambasted Trump in a foreign policy speech made Thursday in California, calling the New York mogul a "fraud" and is "temperamentally unfit" to be a president.

Observers said this was a departure from Clinton's usual stiff public persona, which has been a thorn in her side in her efforts to portray herself as someone who understands the struggles of ordinary Americans.

"Hillary Clinton is displaying more passion in attacking Donald Trump," Brookings Institution's Senior Fellow Darrell West told Xinhua.

The recent fraud scandal over the Trump University gives Clinton an opening to characterize Trump as "a rich guy taking advantage of poor people," West said.

Some Trump University students claimed that they paid to the university a big sum of money but did not get the money's worth.

"That is a script she knows how to follow since that is how Obama beat Romney in 2012," West said, referring to US President Barack Obama's win over Republican challenger Mitt Romney nearly four years ago.

Clinton is likely to continue that theme and extend it to other business dealings of Trump and argue that he is not pro-worker and has taken advantage of many people over the years, West predicted.

If Clinton can turn this into a more conventional race of herself standing up for the little guy and Trump enriching himself at the expense of average people, it moves the campaign onto more familiar turf for her and Democrats in general, West said.

"The race simply would play into common stereotypes about Republicans being out of touch and unfair to working people and it would hurt Trump's ability to win blue-collar support," West said.

A supporter of US Democratic presidential candidate Hillary Clinton wears campaign supporting fingernails at a "Women for Hillary" event in Culver City, California, United States, June 3, 2016. [Photo/Agencies]

Indeed, Clinton called Trump a "fraud", claiming that the Republican nominee is pulling the wool over the eyes of his supporters.

"The fraud argument works well for her because it puts a dagger through the heart of his central claim, that he is a great businessman and that he made his money through smart and effective leadership," West said.

Fraud creates an alternative narrative that says he got his money illicitly and that he cannot be counted upon to help the average person, West said.

Clinton's narrative is meant to counter that of Trump, who has taken to calling Clinton "crooked Hillary," referring to the many scandals she and her husband, former President Bill Clinton, have been tied to over the last 20 years.

Clinton is currently embroiled in a scandal over whether she jeopardized US national security by using a private email account and server for official business while she was secretary of state.

A House panel is also investigating Clinton's responsibility for the 2012 terror attack on the US consulate in Benghazi, Libya, in which four Americans including US Ambassador Chris Stevens were killed, when Clinton was secretary of state.

Just six months ago, most political observers dismissed Trump as a flash in the pan, but he now stands out as the winner in the Republican nomination fight. And in many recent polls, Trump is tied with Clinton in the lead up to what experts said is sure to be a tight race.

Dan Mahaffee, an analyst with the Center for the Study of the Presidency and Congress, told Xinhua that much of the Republican establishment and voters are beginning to unify behind Trump despite concerns about his specific policies.

Indeed, House Speaker Paul Ryan, the top Republican politician, said Thursday that he will vote for Trump despite his initial reluctance to endorse the brash businessmen who is disliked by many of the Republican establishment politicians.