The Democratic field is beginning to take shape for the 2020 election, as three major candidates have declared their intention to run for the White House. After Sen. Elizabeth Warren of Massachusetts threw her hat into the race, Rep. Tulsi Gabbard of Hawaii and former secretary of Housing and Urban Development Julian Castro announced their presidential bids in recent days.

The three candidates enter what is expected to be a crowded Democratic primary vying to challenge President Donald Trump or another Republican nominee.

"There are a lot of reasons for me to make this decision," Gabbard told CNN's Van Jones on Friday. "There are a lot of challenges that are facing the American people that I'm concerned about and that I want to help solve."

Gabbard, 37, has served in the House since 2012. Her background and oratory skills have made her a rising star in the party, and her credentials include service in the Iraq War and becoming the first American Samoan and Hindu to be elected to Congress.

She has stated that her top concerns are health care, criminal justice reform and the environment but her main issue is "war and peace."

Castro, 44, who is the identical twin brother of Texas Rep. Joaquin Castro, served as mayor of San Antonio from 2009 to 2014 and has long been viewed as the nation's most ambitious young Latino politician. In 2016, he was on Hillary Clinton's short list for running mate.

"I’m running for president because it’s time for new leadership. Because it’s time for new energy. And it’s time for a new commitment to make sure that the opportunities that I’ve had are available for every American," Castro told a crowd Saturday at San Antonio’s Plaza Guadalupe.

The Stanford and Harvard grad touched on increasing the minimum wage, while making housing and college education more affordable and expanding pre-kindergarten programs. He launched his campaign under the slogan "One nation, one destiny."

Castro, who is seeking to become the first Latino president, has acknowledged he's not a frontrunner.

Warren was once considered a favorite but may face a tough battle from not just Gabbard and Castro but also former Vice President Joe Biden and Robert "Beto" O'Rourke, who is coming off an inspired Texas gubernatorial campaign. Sen. Kamala Harris of California is another prominent candidate who can emerge as a top contender.

According to betting site Bet365, the incumbent Trump remains the favorite at 15/8 odds.

But there are several Democrats who have yet to declare their candidacy and can seriously challenge Trump. Harris and O'Rourke both have 13/2 odds of being elected president, while Biden has 10/1 odds. Warren has been given 16/1 odds and Sen. Bernie Sanders of Vermont has 18/1 odds.

Gabbard and Castro are considered longshots. Gabbard was given 33/1 odds, while Castro is a 100/1 underdog.

No Republican has announced they will run for president. Vice President Mike Pence is a 25/1 underdog to win the election, while former U.S. ambassador to the United Nations Nikki Haley is a 40/1 longshot. Former Ohio Gov. John Kasich has 80/1 odds.

Julian Castro, former U.S. Department of Housing and Urban Development (HUD) Secretary and San Antonio Mayor, announces his candidacy for president in 2020, at Plaza Guadalupe on January 12, 2019 in San Antonio, Texas. If successful, Castro would be the first Hispanic candidate to win the White House. Edward A. Ornelas/Getty Images