Here is a list of the candidates:

DONALD TRUMP

Trump, in office from 2017-2021, has secured enough delegates to clinch the Republican nomination, setting up the first presidential rematch in nearly 70 years.

He has leveraged his unprecedented legal challenges to solidify support among his base and has cast his third bid for the White House in part as retribution against perceived political enemies. He calls supporters jailed for the Jan. 6, 2021, attack on the U.S. Capitol "hostages" and campaigns using increasingly dystopian rhetoric.

Trump, 77, faces 88 charges in four criminal cases over efforts to subvert the 2020 election as well as unlawfully keeping classified national security documents and falsifying business records. His first criminal trial began in New York on April 15. 

He has said, without evidence, that the criminal charges are a Democratic conspiracy designed to keep him from winning, with some of his legal challenges reaching the U.S. Supreme Court. The U.S. Justice Department denies any political interference. 

If elected to another four-year term, Trump has vowed revenge on his political enemies, and said he would not be a dictator except "on day one." He also wants the power to replace federal civil service workers with loyalists.

He sparked criticism from Western leaders for saying the U.S. would not defend NATO members that failed to spend enough on defense and would encourage Russia to attack them. He also pressed congressional Republicans to stall military aid for Ukraine.

Trump has made immigration his top domestic campaign issue, declaring he would carry out mass deportations, create holding camps, utilize the National Guard, end birthright citizenship, and expand a travel ban on people from certain countries. He has referred to migrants as "animals."

On abortion, Trump takes credit for the U.S. Supreme Court ruling overturning Roe v. Wade and in April said abortion should remain a state issue. He has criticized some Republican-led states' six-week abortion bans and has declined to support a federal ban.

He has promised sweeping changes that included eliminating Obamacare health insurance before saying in an April 11 video that he would not "terminate" it. He has also vowed to undo much of the Biden administration's work to fight climate change.

Trump has yet to announce a vice presidential running mate, but several possibilities have been floated. Mike Pence, who ran alongside Trump in 2016 and 2020 but was targeted by Trump and his supporters amid the Jan. 6 attack, refused to endorse him in November's contest.

DEMOCRATIC PARTY

JOE BIDEN

Biden launched his 2020 candidacy as an urgent bid to defend American liberties and protect democracy and has cast his reelection bid in the same light, saying Trump threatens the future of American democracy.

Biden faced no serious challenger for the party's nomination, which he clinched in March.

November's election will be much tougher, with the most recent Reuters/Ipsos poll putting Biden's voter support slightly ahead at 41% of registered voters, 4 percentage points ahead of Trump's 37%.

Biden, the oldest U.S. president ever at 81, must convince voters he is more fit for office than Trump, who is just four years younger, while also combating low approval ratings.

The economy will also factor in his reelection campaign. While the U.S. escaped an anticipated recession and is growing faster than economists expected, inflation and the cost of essentials is weighing on voters. 

Biden pushed through massive economic stimulus and infrastructure spending packages to boost U.S. industrial output, but has received little recognition from voters so far as his campaign moves to highlight new semiconductor manufacturing plants, housing plans, and other economic efforts.

Biden's handling of immigration policy has also been criticized by Republicans and Democrats as migrant crossings at the U.S.-Mexico border hit record highs.

He has led the response of Western governments to Russia's invasion of Ukraine, persuading allies to sanction Russia and support Kyiv. 

Biden has provided military aid to Israel in its conflict with Hamas in Gaza while pushing for more humanitarian assistance, but he has faced sharp criticism from some Democrats for not pushing harder for a ceasefire or matching his tougher rhetoric on Israel with action. He has threatened to condition the aid on Israeli steps to protect aid workers and civilians and said the U.S. will not take part in a counteroffensive against Iran.

MARIANNE WILLIAMSON

Best-selling author and self-help guru Marianne Williamson, 71, relaunched her long-shot 2024 presidential bid focusing on "justice and love" less than one month after dropping out.

In a February statement, she said she was getting back in to fight Trump's "dark and authoritarian vision" after earlier suspending it because she was losing "the horse race." Williamson previously ran as a Democrat in the 2020 presidential primary but dropped out of that race before any votes were cast. 

INDEPENDENTS

ROBERT F. KENNEDY JR

An anti-vaccine activist and environmental advocate, Kennedy, 70, is running as an independent after initially challenging Biden for the Democratic nomination.

While he lags in overall polling, Kennedy could siphon votes from Trump and Biden, with a March Reuters/Ipsos poll showing he was backed by 16% of registered voters.

Kennedy is the son of U.S. Senator Robert F. Kennedy, who was assassinated in 1968 during his own presidential bid, but he has drawn rebukes from his famous family, which also reportedly plans to step up efforts to publicly back Biden.

Kennedy, who chose wealthy lawyer Nicole Shanahan as his running mate. supports Israel and has questioned the six-week ceasefire backed by Biden. He said he views the U.S. southern border situation as a humanitarian crisis and opposes Trump's border wall. He has also vowed to repeal parts of Biden's climate bill over tax breaks he says help the oil industry.

On health, Kennedy has taken different positions on abortion. He has been criticized for making false medical claims over the years on vaccines but says he would still allow Americans to have access to them.

CORNEL WEST

The political activist, philosopher and academic is running as a third-party bid for president that is likely to appeal to progressive, Democratic-leaning voters.

West, 70, initially ran as a Green Party candidate, but in October he said people "want good policies over partisan politics" and announced his bid as an independent. He has promised to end poverty and guarantee housing.

JILL STEIN

Jill Stein, a physician who ran under the Green Party in 2016, launched her current campaign accusing Democrats of betraying their promises "for working people, youth and the climate again and again - while Republicans don't even make such promises in the first place."

Stein, 73, raised millions of dollars for recounts after Trump's surprise 2016 victory. Her allegations yielded only one electoral review in Wisconsin, which showed Trump had won.

(Reporting by Costas Pitas and Susan Heavey; Editing by Ross Colvin, Jonathan Oatis, Lisa Shumaker and Daniel Wallis)