There could be 20 Democrats or more running 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. 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.

Pete Buttigieg, D

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.

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.

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.

Gov. Jay Inslee, D-Wash.

Inslee served 15 years in Congress before becoming Washington state governor in 2013. He's running primarily on a platform of combating climate change and creating a green economy.

Sen. Amy Klobuchar, D-Minn.

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

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.

Sen. Bernie Sanders, D-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.

Sen. Elizabeth Warren, D-Mass.

Warren is an expert on bankruptcy law and is known for her advocacy of protections for consumers. 

Former Gov. Bill Weld, R, Mass.

The former Massachusetts governor announced on Feb. 15, 2019, that he was launching an exploratory committee to challenge President Trump for the Republican nomination. 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 hinted at a run?

Democrats

  • Sen. Michael Bennet, D-Colo.
  • Former Vice President Joe Biden
  • Gov. Steve Bullock, D-Mont.
  • New York City Mayor Bill de Blasio
  • Former Tallahassee, Florida, mayor Andrew Gillum
  • Former Secretary of State John Kerry
  • Former Gov. Terry McAuliffe, D-Md.
  • Rep. Seth Moulton, D-Mass.
  • Rep. Tim Ryan, D-Ohio
  • Rep. Eric Swalwell, D-Calif.

Republicans

Independents

  • Former Starbucks CEO Howard Schultz

Who is already out?

  • Attorney Michael Avenatti, D
  • Former New York City Mayor Michael Bloomberg, D
  • Sen. Sherrod Brown, D-Ohio
  • Former Secretary of State Hillary Clinton, D
  • Former Sen. Jeff Flake, R-Ariz. 
  • Los Angeles Mayor Eric Garcetti, D
  • Former U.S. Attorney General Eric Holder, D
  • Sen. Jeff Merkley, D-Ore.
  • Sen. Chris Murphy, D-Conn.
  • Former Maryland Gov. Martin O'Malley, D
  • West Virginia Senator Richard Ojeda, D (Suspended campaign)
  • Billionaire hedge fund manager Tom Steyer, D
  • Sen. Jon Tester, D-Mont.
  • Oprah Winfrey