FILE - Rep. Tim Ryan, D-Ohio, speaks at the Heartland Forum on the campus of Buena Vista University in Storm Lake, Iowa, March 30, 2019.
FILE - Rep. Tim Ryan, D-Ohio, speaks at the Heartland Forum on the campus of Buena Vista University in Storm Lake, Iowa, March 30, 2019.

U.S. Rep. Tim Ryan of Ohio on Thursday became the latest Democrat to jump into the race for the party's presidential nomination, joining a crowded field vying to challenge Republican Donald Trump in 2020. 

The pool of Democratic candidates for the White House is among the largest and most diverse ever.

It includes female U.S. senators, a current and a former governor, African-Americans, a Hispanic and a young gay mayor, and is likely to grow before the U.S. primary season gets underway next year. 
 
The Democratic nominating convention opens on July 13, 2020, in Milwaukee, Wis. Here are the party's main contenders vying to be on the ballot. 

Tim Ryan

Ryan, 45, was elected to the U.S. House of Representatives from Ohio in November 2002 at age 29. He has won re-election seven times and currently serves on the Appropriations Committee.

Ryan launched his campaign for president on a platform of investing in public education and providing affordable health care. 

A moderate Democrat, Ryan mounted an unsuccessful challenge in 2016 for the Democratic leadership of the House against Rep. Nancy Pelosi of California. 

Beto O'Rourke

During a frenetic if failed campaign for the U.S. Senate last year in Texas, O'Rourke, 46, used his youth, energy and camera-friendly looks to become a media darling while setting fundraising records and drawing support from a range of celebrities. 
 
Despite a reputation forged in his three terms in Congress as a pragmatic centrist, O'Rourke launched his campaign in Iowa on a decidedly left-leaning platform, calling for health and immigration reform, a higher minimum wage and an all-out battle to curb climate change. 
 
His resolutely positive message, with calls for "kindness and decency" — along with criticisms of an "unfair, unjust and racist capitalist economy" — have drawn large and often youthful crowds wherever he appears.

FILE - Sen. Kirsten Gillibrand, a New York Democra
FILE - Sen. Kirsten Gillibrand, D-N.Y., speaks during the New York Senate debate hosted by WABC-TV, in New York, Oct. 25, 2018.

?Kirsten Gillibrand

The New York senator, 52, had made a name campaigning against sexual abuse, especially in the military, even before the #MeToo movement gained national prominence. A fierce Trump critic, Gillibrand is making gender and women's issues a hallmark of her campaign. 
 
She has called for a more egalitarian society and wants to improve the nation's health and education systems.

Bernie Sanders

The self-described democratic socialist, 77, was an outsider when the 2016 Democratic primaries began. But he gave favorite Hillary Clinton a run for her money with his calls for a "political revolution" and battled her down to the wire. 
 
Sanders won passionate support among young liberals with his calls for universal health care, a $15 minimum wage and free public university education.

Democratic presidential candidate Sen. Amy Klobuch
FILE - Democratic presidential candidate Sen. Amy Klobuchar, D-Minn., speaks at the Heartland Forum on the campus of Buena Vista University in Storm Lake, Iowa, March 30, 2019.

?Amy Klobuchar

The 58-year-old granddaughter of an iron miner, Klobuchar is a former prosecutor with an unpretentious demeanor. 
 
She has quietly gained attention in Washington as a centrist. Klobuchar is known for putting partisanship aside to pass legislation, something that has earned her a devoted following in Minnesota. 
 
Klobuchar has promised more stringent gun laws and set a target of universal health care.

Elizabeth Warren

At 69, the U.S. Senate's consumer protection champion from Massachusetts is on the party's left flank. She built her reputation by holding Wall Street accountable for its missteps.  
 
Warren is considered to have one of the best campaign organizations of any Democrat. Her campaign has been dogged, however, by her past claims of Native American heritage, and Trump mockingly refers to her as "Pocahontas."

Cory Booker

The New Jersey senator, 49, announced his candidacy Feb. 1, evoking the civil rights movement as he promised to work to unite a divided America. 
 
Often compared to former President Barack Obama, Booker began his career as a community activist and rose to prominence as mayor of Newark, N.J. 
 
A talented orator, Booker was elected to the U.S. Senate in 2013, the first African-American senator from the Eastern state.

Kamala Harris

The barrier-breaking senator from California who aspires to be the nation's first black female president announced her candidacy on a day honoring slain civil rights icon Martin Luther King Jr. 
 
The daughter of an Indian immigrant medical researcher mother and a Jamaican economist father, Harris, 54, began her career as a district attorney in San Francisco before serving as California's attorney general.

FILE - Pete Buttigieg, mayor of South Bend, Indian
FILE - Pete Buttigieg, mayor of South Bend, Ind., talks with a reporter in South Bend, Jan. 10, 2019.

?Pete Buttigieg

The South Bend, Ind., mayor, 37, joined the race with a resolutely forward-looking and optimistic message to counter Trump's darker vision. 
 
A Rhodes Scholar, Buttigieg would be the first openly gay presidential nominee of either major party. 
 
A U.S. Navy Reserve officer, he put his mayoral duties aside to serve in Afghanistan in 2014.

Other candidates

Also in the race are former San Antonio Mayor Julian Castro, 44; U.S. Rep. John Delaney of Maryland, 55; U.S. Rep. Tulsi Gabbard, 37, of Hawaii; former Colorado Gov. John Hickenlooper, 67; Jay Inslee, 68, the governor of Washington state; Wayne Messam, 44, the mayor of Miramar, Fla.; self-help author Marianne Williamson, 66; and technology executive Andrew Yang, 44.  

Waiting in the wings

Among the big Democratic guns who have yet to commit is former Vice President Joe Biden, who leads most surveys of Democratic voters. 
 
Biden, who combines experience and widespread popularity, would be expected to poll well in some of the blue-collar Midwestern states that propelled Trump to the presidency in 2016.