First-term Democratic Senator Kamala Harris of California, a rising party star and outspoken critic of President Donald Trump's immigration policies, launched her 2020 campaign for the White House on Monday by touting her prior career as a prosecutor.

Harris, 54, the daughter of immigrants from Jamaica and India, enters the race with the potential advantage of being the Democratic candidate who looks most like the party's increasingly diverse base of young, female and minority voters.

"I have the unique experience of having been a leader in local government, state government and federal government," she said in an appearance on ABC's Good Morning America.

She timed her announcement for the US Martin Luther King Jr Day holiday honouring the slain civil rights leader, saying he was an aspirational leader.

"We are the best of who we are when we fight to achieve these ideals," she said on ABC. "We are flawed, we are not perfect, but we are a great country when we think about the principles upon which we're founded."

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The former California state attorney general has become popular with liberal activists for her tough questioning of Trump administration appointees and officials, including Supreme Court nominee Brett Kavanaugh and former Attorney General Jeff Sessions, during Senate hearings.

Her campaign will focus on reducing the high cost of living with a middle-class tax credit, pursuing immigration and criminal justice changes and a Medicare-for-all healthcare system. She has said she will reject corporate political action committee money.

Trump critic

Harris's campaign will be based in Baltimore, with a second office in Oakland, California. Her slogan will be "For the People", in a nod to Harris's roots as a prosecutor, aides said.

She will hold a launch rally in Oakland before the end of the month.

As one of the earliest congressional critics of Trump's immigration policies, Harris has pushed hard for a deal to protect from deportation those immigrants who came to the country without documents as children, a group known as Dreamers.

Harris enters a field that is shaping up to be a crowded battle for the nomination to challenge Trump, the likely Republican candidate.

She and other Democrats will have to navigate the party's debate about whether an establishment figure who can appeal to centrist voters or a fresh face who can energise its increasingly diverse and progressive base offers the best chance to beat Trump in 2020.

Harris, who made history in 2016 as the first black woman elected to the US Senate from California, has embraced the party's diversity ahead of a Democratic nominating campaign where minority voters and liberal activists are expected to have an outsized voice.

She has pushed back against critics of "identity politics", who she says are using the term as a pejorative to marginalise issues of race, gender and sexual orientation.

"It is used to try and shut us up," Harris told a conference of liberal activists last summer.

The former San Francisco prosecutor drew notice when her rapid-fire grilling of Sessions during a 2017 Senate Judiciary Committee hearing caused him to complain.

"It makes me nervous," Sessions said.

Her campaign could be aided by the schedule for the state-by-state party nominating process that is scheduled to begin in February 2020.

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The kickoff state of Iowa, which launched former President Barack Obama's presidential bid in 2008, has a strong base of liberal activists, and the race will then quickly move to more diverse states such as Nevada and South Carolina. Her home state of California also has moved up its primary to increase its influence.

In September, she was among a handful of Democrats who aggressively questioned Kavanaugh at his Supreme Court confirmation hearing about his views on abortion and on the special counsel probe into potential Russian meddling in the 2016 election.

In the Senate, she has introduced a bill to give lower-income families cash payments and tax credits to help battle wage stagnation and rising housing costs, and has been a strong advocate of criminal justice reforms.

'Wrong side of history'

Harris launched a book tour in early January to promote a memoir, making a series of media appearances that helped bolster her visibility ahead of her campaign announcement.

In the memoir, Harris described herself as a "progressive prosecutor" and said it's a "false choice" to decide between supporting the police and advocating for greater scrutiny of law enforcement.

The argument is aimed at liberal critics of her record who argue she was sometimes too quick to side with the police and too slow to adopt sentencing reforms.

But Harris has fielded some criticism from both right-wing supporters of Trump and progressives to the left of her.

In a New York Times op-ed, University of San Francisco Law Professor Lara Bazelon argued that Harris was "often on the wrong side of history".

Many progressives have criticised her record, including her sometimes reluctant approach to criminal justice reform and her fighting against lawsuits brought by sex workers and opponents of solitary confinement in prisons.

Political foes will pour over her record in California, where she has come under scrutiny for declining as attorney general to prosecute OneWest, the bank once headed by Treasury Secretary Steven Mnuchin, for alleged foreclosure violations.

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Harris, who voted against Mnuchin's confirmation as head of the Treasury, has said she "followed the facts" in declining to prosecute.

She also has been criticised for saying she was not aware of sexual harassment allegations against one of her top aides, who resigned in December after a California newspaper asked him about a 2016 harassment lawsuit.

Xavier Becerra, who replaced Harris as attorney general, settled the lawsuit in May 2017 for $400,000.