More than 20 Democrats have declared to run for their party's presidential nomination to take on President Donald Trump in 2020. But Trump is also facing a challenge from within his own party, something which is not unprecedented for a sitting president but has succeeded only once.

Here is a look at who has announced they are running, who is hinting at it, and who has already decided to take a pass.

Who is running?

President Donald Trump, R

The president launched his reelection campaign on the day of his inauguration. While Trump has high popularity among Republicans, he has yet to break an average 50 percent overall approval rating in most polls.

Sen. Michael Bennet, D-Colo.

Bennet is a former head of Denver Public Schools who has carved out a reputation as a policy-oriented moderate. He's pushed back on the single-payer health care and instead proposes letting consumers buy into Medicare through insurance exchanges.

Former Vice President Joe Biden, D

This is Biden's third run for the presidency. He was elected to seven terms in the U.S. Senate before serving as Barack Obama's vice president. Biden has said he would campaign as an "Obama-Biden Democrat," who is as pragmatic as he is progressive.

Sen. Cory Booker, D-NJ

Booker is a former mayor of Newark, New Jersey. Known for answering residents' call for help during a snowstorm by shoveling snow himself and directing plows, and starting a nonprofit to help residents transform their neighborhoods.

Gov. Steve Bullock, D-Mont.

Bullock won re-election as governor in 2016 in a state that Trump won by 21 points. Signed a law requiring anonymous groups to report campaign contributions in state elections.

Mayor Pete Buttigieg, D-South Bend, Ind.

Buttigieg was elected mayor of South Bend, Indiana, at the age of 29. He's a lieutenant in the Navy Reserve and served a tour in Afghanistan. He could become the first openly gay presidential nominee from a major political party.

Julián Castro, D

The former San Antonio mayor and U.S. Housing and Urban Development secretary was the first Latino to give the keynote address at the Democratic National Convention.

Mayor Bill de Blasio, D-New York City

As mayor, de Blasio sought reform of the city’s “stop and frisk” policy and pushed for more affordable housing. He supported paying for universal pre-kindergarten by taxing those making over $500,000 a year.

Former Rep. John K. Delaney, D-Md.

The former U.S. congressman from Maryland chose not to run for re-election in 2018 so that he could focus on running for the White House. Known for willingness to work with Republicans while in Congress.

Rep. Tulsi Gabbard, D-Hawaii

Gabbard is a major in the U.S. Army National Guard. She faces controversy over previously held anti-gay views but says she is now an LGBT supporter.

Sen. Kirsten Gillibrand, D-NY

Over the past decade, some of Gillibrand's policy positions have shifted from conservative to more liberal as she moved from being a representative in a more conservative district to a senator for all of New York state.

Sen. Kamala Harris, D-Calif.

Harris, whose father is Jamaican and mother is Indian, announced her candidacy on Martin Luther King Jr. Day. Harris is a former California attorney general in her first term as a U.S. senator.

Sen. Amy Klobuchar, D-Minn.

Klobuchar, who graduated magna cum laude from Yale, is known for reaching across the aisle to get things done. 

Mayor Wayne Messam, D-Miramar, Fla.

As the son of Jamaican immigrants, the Miramar, Fla., mayor says his story will resonate with voters. Messam said he wants universal health care, student loan forgiveness, infrastructure improvements, and focus education toward a high-tech future.

Rep. Seth Moulton, D-Mass.

A veteran of the Iraq war. Despite occasionally differing with some on the most liberal wing of the party, Moulton has staked out familiar policy positions for those seeking the nomination including health care as a right and co-sponsoring the Green New Deal.

Former Rep. Beto O'Rourke, D-Texas

In the 2018 midterms, O'Rourke got within 3 percentage points of upsetting Republican Sen. Ted Cruz in the nation's largest red state — and shattered national fundraising records in the process.

Rep. Tim Ryan, D-Ohio

The representative from Ohio has resisted being labeled a political centrist, receiving backing from the National Rifle Association and reversing his past opposition to abortion in favor of abortion rights.

Sen. Bernie Sanders, I-Vt.

Sanders finished second to Hillary Clinton for the Democratic nomination in 2016. Despite running as a Democrat, Sanders has labeled himself as an independent throughout his political career.

Former Rep. Joe Sestak, D-Pa.

Sestak joined the Democratic field on June 23, three days before the first primary debate. He's a retired three-start Navy admiral and a former two-term congressman from Pennsylvania.

Tom Steyer, D

The billionaire has run an ongoing ad campaign calling for President Donald Trump's impeachment. He entered the race in July, six months after announcing he would not seek the nomination.

Sen. Elizabeth Warren, D-Mass.

Warren is an expert on bankruptcy law and is known for her advocacy of protections for consumers. Has proposed a wealth tax and a massive program to forgive student loan debt.

Former Gov. Bill Weld, R, Mass.

The self-described Reagan Republican has called on President Trump to resign since the release of Robert Mueller's report into the 2016 election. Weld was the 2016 Libertarian vice presidential nominee.

Marianne Williamson, D

The spiritual leader, author and activist says on her website that America needs to "address the deep emotional and psychological dynamics within the average citizen that have led to the erosion of our political system."

Andrew Yang, D

An entrepreneur, Yang's primary campaign platform is a Universal Basic Income in which the government would pay $12,000 per year to each citizen over age 18.

Who has dropped out?

Former Sen. Mike Gravel, D

Gravel didn't want to win the nomination. The former two-term U.S. Senator from Alaska said he just wanted to make it to the debate stage to push the platform to the left. He didn't qualify for either of the first two debates. He ended his campaign on Aug. 6, 2019, and endorsed Bernie Sanders.

RELATED: Meet the Democratic presidential candidate who isn't trying to win

Former Gov. John Hickenlooper, D-Colo.

Hickenlooper served two terms as Colorado's governor which included becoming the one of the first states to make recreational marijuana use legal -- something he disagreed with. Helped Denver eliminate $70 million in debt as mayor. Hickenlooper officially ended his presidential campaign on Aug. 15, 2019. 

RELATED: John Hickenlooper ends presidential campaign

Gov. Jay Inslee, D-Wash.

Inslee served 15 years in Congress before becoming Washington state governor in 2013. He ran primarily on a platform of combating climate change and creating a green economy. Inslee dropped out of the race on Aug. 21, 2019, indicating he knew he would not win the nomination.

RELATED: Washington Gov. Jay Inslee ends presidential bid

Richard Ojeda, D

The West Virginia state senator's campaign lasted two months, ending in January 2019. Ojeda told supporters he could not continue to ask them for donations when it was clear he was getting no traction.

RELATED: Richard Ojeda exits from 2020 presidential race

Rep. Eric Swalwell, D-Calif.

The California congressman, whose key platform was gun control, ended his campaign on July 8, 2019, after failing to gain traction following his first and only debate.

RELATED: Eric Swalwell drops out of presidential race

Who has hinted at a run?

Republicans

  • Former Gov. John Kasich, R-Ohio

Independents

  • Rep. Justin Amash, Mich. (Amash left the Republican party on July 4, weeks after becoming the first GOP member of Congress to call for impeachment proceedings against President Donald Trump.)
  • Former Starbucks CEO Howard Schultz
Obama
The presidential seal on a podium before a speech by President Barack Obama on a U.S. counterterrorism strategy speech at MacDill Air Force Base Tuesday, Dec. 6, 2016, in Tampa, Fla. (AP Photo/Chris O'Meara)
AP