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 has announced they are 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.

Juli├ín Castro, D

The former San Antonio mayor and U.S. Housing and Urban Development secretary announced on Jan. 12, 2019. He was the first Latino to give the keynote address at the Democratic National Convention.

John K. Delaney, D

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. He announced his candidacy in July 2018.

Rep. Tulsi Gabbard, D-Hawaii

Gabbard's announcement came on CNN on Jan. 11, 2019. She 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

Gillibrand announced on Jan. 15, 2019, that she was launching an exploratory committee. Over the past decade, some of her policy positions have shifted from conservative to more liberal.

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. She entered the race on Feb. 10.

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. She formally joined the race Feb. 9.

Bill Weld, R

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.

Who has hinted at a run?

Democrats

  • Sen. Michael Bennet, D-Colo.
  • Former Vice President Joe Biden
  • Former New York City Mayor Michael Bloomberg
  • Sen. Sherrod Brown, D-Ohio
  • Gov. Steve Bullock, D-Mont.
  • New York City Mayor Bill de Blasio
  • Former Tallahassee, Florida, mayor Andrew Gillum
  • Former Gov. John Hickenlooper, D-Colo.
  • Former U.S. Attorney General Eric Holder
  • Gov. Jay Inslee, D-Wash.
  • Former Secretary of State John Kerry
  • Former Gov. Terry McAuliffe, D-Md.
  • Sen. Jeff Merkley, D-Ore.
  • Former Rep. Beto O'Rourke, D-Texas
  • Rep. Tim Ryan, D-Ohio
  • Rep. Eric Swalwell, D-Calif.

Republicans

Independents

  • Former Starbucks CEO Howard Schultz

Who is already out?

  • Attorney Michael Avenatti
  • Former Sen. Jeff Flake, R-Ariz. 
  • Los Angeles Mayor Eric Garcetti
  • Former Maryland Gov. Martin O'Malley, D
  • Sen. Chris Murphy, D-Conn.
  • West Virginia Senator Richard Ojeda, D (Suspended campaign January 25, 2019)
  • Billionaire hedge fund manager Tom Steyer, D
  • Sen. Jon Tester, D-Mont.
  • Oprah Winfrey