WASHINGTON (Reuters) - U.S. Democrats, anxious to get started on their quest to recapture the White House in 2020, are lining up to launch campaigns in what is quickly becoming a crowded field of presidential contenders.

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A diverse group that includes at least six U.S. senators will be vying for the right to challenge President Donald Trump, the likely Republican nominee, who could face a long-shot primary challenger.

Here are the Democrats who have either launched campaigns or formed exploratory committees to begin raising money and hiring staff for a presidential run:

CORY BOOKER - Booker, 49, a black second-term senator from New Jersey and former mayor of Newark, gained prominence in the fight over Brett Kavanaugh’s Supreme Court nomination. He announced his candidacy on Feb. 1, the first day of Black History Month, mentioning the impact of racial discrimination on his family and saying he would focus on creating good jobs and reforming the criminal justice system.

PETE BUTTIGIEG - The mayor of South Bend, Indiana, launched a campaign to become the first openly gay president on Jan. 23 with a call for a new generation of leadership to bring fresh approaches to problems. Buttigieg, 37, brushed off questions about his experience by highlighting the economic turnaround in his city.

JULIAN CASTRO - The secretary of housing and urban development under President Barack Obama joined the race onJan. 12 in his hometown of San Antonio, Texas, where he once served as mayor and a city councilman. Castro, 44, the grandson of a Mexican immigrant, is so far the only Hispanic in the field. He has used his family’s personal story to criticize Trump’s border policies.

JOHN DELANEY - The former U.S. representative from Maryland was the first Democrat to enter the race in July 2017, long before most candidates began making their moves. Delaney, 55, has focused heavily on campaigning and organizing in Iowa, the state that will kick off the nominating fight in February 2020,making repeated trips there and visiting all 99 counties.

TULSI GABBARD - Gabbard, 37, the first Hindu to serve in the U.S. House of Representatives, announced she would run on Jan.11. Her campaign quickly became engulfed in controversy over her past anti-gay activism and statements, and the Hawaii congresswoman was forced to apologize. “In my past, I said and believed things that were wrong,” she said.

KIRSTEN GILLIBRAND - An outspoken leader in the #MeToomovement against sexual assault and harassment, the senator from New York announced her candidacy on Jan. 15 on “The Late Show with Stephen Colbert.” Gillibrand, 52, touted her rural roots in upstate New York on a subsequent trip to Iowa and said she had proven her ability to win over more conservative rural voters.

KAMALA HARRIS - She announced her candidacy on the holiday honoring slain civil rights leader, Martin Luther King Jr. The daughter of immigrants from Jamaica and India has made a quick impact in a Democratic race that will be heavily influenced by women and minority voters. The 54-year-old, first-term senator from California reported raising $1.5 million online in the first 24 hours of her campaign and drew record ratings on a CNN televised town hall.

JOHN HICKENLOOPER - The 67-year-old former Colorado governor has positioned himself as a centrist and an experienced officeholder with business experience. During his two terms in office, Colorado’s economy soared and the Western state expanded healthcare, passed a gun control law and legalized marijuana. The former geologist and brew pub owner is one of two governors in the race and so far has refused to take corporate money for his political action committee.

JAY INSLEE - The Washington state governor entered the field on March 1 with a vow to make climate change a central issue. Inslee, 68, has made protecting the environment a key fixture of his administration. He has moved to put a moratorium on capital punishment and fully implement the Affordable Care Act and accompanying expansion of Medicaid health coverage for the poor. He spent 15 years in Congress before being elected governor in 2012.

AMY KLOBUCHAR - She was the first moderate in the field vying to challenge Trump. Klobuchar, 58, gained national attention in 2018 when she sparred with Brett Kavanaugh during Senate hearings on his Supreme Court nomination. Now in her third six-year term as a senator for Minnesota, Klobuchar’s campaign announcement came amid news reports that staff in her Senate office were asked to do menial tasks, making it difficult to hire high-level campaign strategists.

BERNIE SANDERS - The Vermont senator, 77, lost the Democratic nomination in 2016 to Hilary Clinton. Sanders drew strong backing from younger voters who supported his proposals for free tuition at public colleges, a $15 minimum wage and universal healthcare, ideas he plans to reprise as he vies for the 2020 nomination. The campaign reported raising $10 million in the first week of his run.

ELIZABETH WARREN - The 69-year-old senator from Massachusetts, a leader of the party’s liberals and fierce Wall Street critic who was instrumental in creating the Consumer Financial Protection Bureau, jumped into the race with a New Year’s Eve video release. On visits to states that hold early nominating contests such as Iowa and New Hampshire, she has focused on her populist economic message, promising to fight what she calls a rigged economic system that favors the wealthy.