Buoyed by Tuesday's takeover of the US House of Representatives, Democrats can now turn their attention to the 2020 presidential race.

For the first time since the start of the 2004 campaign, Democrats are entering the cycle without a dominant front-runner. More than two dozen possible contenders have had their names floated or have actively begun exploring their chances.

US President Donald Trump filed for re-election the day he was inaugurated in January 2017, and his popularity with the Republican Party's core supporters means any possible challenge for the party's nomination will be a long shot.

Here are some of the potential contenders in each party:

Democrats:

Joe Biden

The former vice president, 75, is the early Democratic leader in polls, and that is partly a function of familiarity given his decades as a senator and eight years as a No 2 to Barack Obama.

If he makes his third run for the presidency, Mr Biden will have easy access to top-shelf staff, donors and an extensive network of supporters. His age could work against him in a party looking for fresher faces, and his ties with the former president would make him an easy target for Republican attacks.

Bernie Sanders

The Vermont senator, 77, still has a loyal following from his 2016 challenge to Hillary Clinton, and his focus on issues such as universal healthcare, reducing income inequality and tuition-free public college has been adopted widely by the party.

But while he was an insurgent candidate two years ago, he would face more intense scrutiny as a major contender in 2020.

Elizabeth Warren

The Massachusetts senator, 69, is a leader of the party's progressives and a fierce critic of Wall Street who was instrumental in creating the Consumer Financial Protection Bureau.

Her recent decision to take a DNA test to prove her distant Native American ancestry after Donald Trump's taunts of "Pocahontas" was roundly criticised and raised questions among some Democrats about her political agility.

Kamala Harris

The first-term senator from California, 54, is considered one of the candidates most likely to break out from the pack of lesser known Democrats.

Her aggressive questioning of Supreme Court nominee Brett Kavanaugh and her decision to swear off corporate PAC money – large donations from businesses - won her plaudits from activists.

But as a newcomer to national politics, she still needs to introduce herself to the public while defying Republican attempts to define her negatively.

Cory Booker

The two-term senator from New Jersey, 49, a former Rhodes Scholar and Stanford University football player, won notice as the mayor of Newark, New Jersey, when he saved a neighbour from a burning house in 2012.

Some liberals have criticised him for having close ties to Wall Street and for helping to kill a proposal that would have lowered prescription drug prices.

Andrew Cuomo

The New York governor, 60, easily defeated a primary challenge from the left by actress Cynthia Nixon in September.

A big re-election win makes him a possible contender.

Eric Holder

A close ally of Barack Obama, he served as his first attorney general and has launched a committee to fight battles over redistricting, the drawing of district lines that can cement a party's hold on power.

Mr Holder, 67, drew rebukes from Republicans, and some groans from Democrats, when he said in October of Republicans: "When they go low, we kick them. That's what this new Democratic Party is about."

He later told critics to "stop the fake outrage."

Amy Klobuchar

The two-term senator from Minnesota, 58, a former prosecutor, won praise from activists for her questioning of Supreme Court nominee Brett Kavanaugh at his confirmation hearing.

He memorably turned the question back to her when she asked if he had blacked out from drinking. "I don't know, have you?" he asked Klobuchar, who had revealed that her 90-year-old father was a recovering alcoholic. Kavanaugh later apologised.

Terry McAuliffe

Like Joe Biden, the former Virginia governor has broad access to donors and influential Democrats.

Mr McAuliffe, 61, is a former chairman of presidential campaigns for both Bill and Hillary Clinton, and a former head of the Democratic National Committee from 2001 to 2005.

Steve Bullock

The Montana governor, 52, has asserted an interest in running for president with multiple trips to early primary states, including a well-publicised trip to the Iowa State Fair.

He has emphasized the need for a national 50-state campaign, saying as a governor he knows how to reach across the aisle to get things done.

Republicans:

Donald Trump

The current president, 72, already has a campaign slogan, "Keep America Great," and between his campaign committee and two joint fundraising committees has raised $106 million for his re-election, with $47 million cash-on-hand, according to campaign finance reports.

He has turned his attention to the race, punctuating his political rallies with frequent put-downs of his possible Democratic rivals.

John Kasich

After a failed presidential campaign in 2016, the Ohio governor has become one of the party's few notable critics of Donald Trump.

Mr Kasich, 66, a moderate on some social issues, has pointedly refused to rule out a primary challenge to the president.

But he is famous for his aversion to fundraising, which could make success elusive.

Concerns about potential primary opposition from Mr Trump's base have encouraged Mr Kasich's allies to view him as a possible independent candidate.

Jeff Flake

The conservative first-term Arizona senator, 55, declined to seek re-election after becoming one of the leading Republican critics of Mr Trump.

He has criticised his fellow Republicans in Congress for failing to stand up to the president.

But he would have difficulty gaining traction in Republican primaries, which Mr Trump's loyal supporters could dominate.