WASHINGTON (Reuters) - The largest group of Democratic presidential candidates in the modern U.S. political era has gotten a little bit smaller while the Republican field slowly grows.

On Sunday, former congressman Joe Walsh became the second primary challenger to launch a campaign against Republican President Donald Trump.

Twenty-one Democratic rivals are now seeking their party’s 2020 nomination, after Massachusetts congressman Seth Moulton abandoned his campaign on Friday.

On Wednesday, Washington state Governor Jay Inslee dropped out, less than a week after former Colorado Governor John Hickenlooper ended his run. U.S. Representative Eric Swalwell of California in July was the first Democratic candidate to exit the contest.

The diverse group of Democrats vying to challenge Trump, who remains the likely Republican nominee, in November 2020 includes seven U.S. senators. A record six women are running, as well as black, Hispanic and openly gay candidates who would make history if one of them becomes the party’s nominee.

A top tier of contenders has emerged from the vast field, while others are still trying to break through.

DEMOCRATIC TOP TIER

Here are the Democrats who are ranked in the top seven in the RealClearPolitics national polling average:

JOE BIDEN

The leader in opinion polls among Democratic presidential contenders, Biden waited until late April to enter the race, launching his bid with a direct swipe at Trump. Biden, 76, served eight years as vice president under President Barack Obama and 36 years in the U.S. Senate. He stands at the center of Democratic debate over whether the party’s standard-bearer should be a veteran politician or a newcomer, and whether a liberal or a moderate has a better chance of defeating Trump. Biden, who frequently notes his “Middle-Class Joe” nickname, touts his working-class roots and his ability to work in a bipartisan fashion. He has faced criticism from some fellow Democrats for his role in passing tough-on-crime legislation in the 1990s.

BERNIE SANDERS

The senator from Vermont lost the Democratic nomination in 2016 to Hillary Clinton but is trying again. For the 2020 race, Sanders, 77, is fighting to stand out in a packed field of progressives running on issues he brought into the Democratic Party mainstream four years ago. His proposals include free tuition at public colleges, a $15 minimum wage and universal healthcare. He benefits from strong name recognition and an unmatched network of small-dollar donors.

ELIZABETH WARREN

The 70-year-old senator from Massachusetts is a leader of the party’s liberals and a fierce critic of Wall Street who was instrumental in creating the federal Consumer Financial Protection Bureau after the 2008 financial crisis. She has focused her presidential campaign on a populist economic message, promising to fight what she calls a rigged system that favors the wealthy. She has released a dizzying array of policy proposals on everything from breaking up tech companies to implementing a “wealth tax” on the richest Americans. Warren has sworn off political fundraising events to back her campaign.

KAMALA HARRIS

The first-term U.S. senator from California would make history as the first black woman to gain the nomination. Harris, 54, the daughter of immigrants from Jamaica and India, announced her candidacy on the holiday honoring slain civil rights leader Martin Luther King Jr. She supports a middle-class tax credit, the Green New Deal and the legalization of marijuana. Her track record as San Francisco’s district attorney and California’s attorney general has drawn scrutiny in a Democratic Party that has grown more liberal in recent years on criminal justice issues. She saw a significant bounce in the polls after a high-profile clash with Biden over race issues during the first Democratic debate but has since seen her numbers drop back down.

PETE BUTTIGIEG

The 37-year-old mayor of South Bend, Indiana, emerged from virtual anonymity to become one of the party’s brightest stars, building momentum with young voters. A Harvard University graduate and Rhodes Scholar at the University of Oxford, he speaks seven languages conversationally and served in Afghanistan with the U.S. Navy Reserve. He touts himself as representing a new generation of leadership needed to combat Trump. Buttigieg would be the first openly gay presidential nominee of a major American political party.

BETO O’ROURKE

The former U.S. congressman from Texas gained fame last year for his record fundraising and ability to draw crowds ahead of his unexpectedly narrow loss in the U.S. Senate race against Republican incumbent Ted Cruz. But with progressive policies and diversity at the forefront of the party’s White House nominating battle, O’Rourke, 46, has faced a challenge as a white man who is more moderate on several key issues than many of his competitors. He has increasingly turned his attention to Trump’s rhetoric about immigrants after a gunman targeting Hispanics killed 22 people on Aug. 3 in El Paso, O’Rourke’s hometown.

CORY BOOKER

Booker, 50, a U.S. senator from New Jersey and former mayor of Newark, gained national prominence in the fight over Brett Kavanaugh’s Supreme Court nomination. Booker, who is black, has made U.S. race relations and racial disparities in the criminal justice system a focus of his campaign. He embraces progressive positions on Medicare coverage for every American, the Green New Deal and other key issues, and touts his style of positivity over attacks.

TRYING TO BREAK THROUGH

The field also includes many Democrats who are looking for a way to break through. Some hold public office and have managed to generate an early fundraising base, while others are still trying to raise their profile.

JULIAN CASTRO

The secretary of housing and urban development under President Barack Obama would be the first Hispanic to win a major U.S. party’s presidential nomination. Castro, 44, whose grandmother immigrated to Texas from Mexico, has used his family’s personal story to criticize Trump’s border policies. Castro advocates universal prekindergarten, supports Medicare for All and cites his experience to push for affordable housing. He announced his bid in his hometown of San Antonio, where he once served as mayor and a city councilman.

AMY KLOBUCHAR

The U.S. senator from Minnesota was the first moderate in the Democratic field vying to challenge Trump. Klobuchar, 59, also gained national attention when she sparred with Brett Kavanaugh during his Supreme Court nomination hearings. On the campaign trail, the former prosecutor and corporate attorney supports an alternative to traditional Medicare healthcare funding and is taking a hard stance against rising prescription drug prices.

ANDREW YANG

The entrepreneur and former tech executive is focusing his campaign on an ambitious universal income plan. Yang, 44, wants to guarantee all Americans between the ages of 18 and 64 a $1,000 check every month. The son of immigrants from Taiwan, Yang supports Medicare for All and has warned that automation is the biggest threat facing American workers. His campaign has released more than 100 policy ideas, including eclectic proposals like creating an infrastructure force called the Legion of Builders and Destroyers. He lives in New York.

TULSI GABBARD

The Samoan-American congresswoman from Hawaii and Iraq war veteran is the first Hindu to serve in the U.S. House of Representatives and has centered her campaign on her anti-war stance. After working for her father’s anti-gay advocacy group and drafting relevant legislation, she was forced to apologize for her past views on same-sex marriage. Gabbard, 38, slammed Trump for standing by Saudi Arabia after the murder of journalist Jamal Khashoggi.

MICHAEL BENNET

Bennet, 54, a U.S. senator for Colorado, has based his political career on improving the American education system. He previously ran Denver’s public schools. Bennet is not well known nationally but has built a network of political operatives and donors helping elect other Democrats to the Senate. During the partial U.S. government shutdown in January, he garnered national attention criticizing Republicans for stopping the flow of emergency funds to Colorado.

STEVE BULLOCK

The Democratic governor of Montana, re-elected in 2016 in a conservative state that Trump carried by 20 percentage points, has touted his electability and ability to work across party lines. Bullock, 53, has made campaign finance reform a cornerstone of his agenda. He emphasizes his success in forging compromises with the Republican-led state legislature on bills to expand Medicaid, increase campaign finance disclosures, bolster pay equity for women and protect public lands.

BILL DE BLASIO

The New York City mayor emerged as a progressive standard-bearer in 2013, when he won his first term running America’s largest city by population on a platform of addressing income inequality. But he has struggled amid middling approval ratings and some political setbacks to build a national profile. De Blasio, 58, can point to a number of policy wins in New York, including universal prekindergarten, a higher minimum wage and paid sick leave. He has called Trump a “bully” and a “con artist” and criticized his administration’s positions on immigration, climate change and social welfare.

KIRSTEN GILLIBRAND

Gillibrand, known as a moderate when she served as a congresswoman from upstate New York, has refashioned herself into a staunch progressive, calling for strict gun laws and supporting the Green New Deal. The 52-year-old U.S. senator for New York has led efforts to address sexual assault in the military and on college campuses, and she pushed for Congress to improve its own handling of sexual misconduct allegations. On the campaign trail, she has made fiery denunciations of Trump.

TIM RYAN

The moderate nine-term congressman from a working-class district in the battleground state of Ohio has touted his appeal to the blue-collar voters who fled to Trump in 2016. Ryan, 46, pledges to create jobs in new technologies and focus on public education and access to affordable healthcare. He first gained national attention when he unsuccessfully tried to unseat Nancy Pelosi as the House Democratic leader in 2016, arguing it was time for new leadership. A former college football player, he also has written books on meditation and healthy eating.

JOHN DELANEY

The former U.S. representative from Maryland became the first Democrat to enter the 2020 race, declaring his candidacy in July 2017. Delaney, 56, says that if elected, he would focus on advancing only bipartisan bills during the first 100 days of his presidency. He is also pushing for a universal healthcare system, raising the federal minimum wage and passing gun safety legislation. A former business executive, Delaney is self-funding much of his campaign.

MARIANNE WILLIAMSON

The 67-year-old New York Times best-selling author, motivational speaker and Texas native believes her spirituality-focused campaign can heal the United States. A 1992 interview on Oprah Winfrey’s show led Williamson to make a name for herself as a “spiritual guide” for Hollywood and a self-help expert. She is calling for $100 billion in reparations for slavery over 10 years, gun control, education reform and equal rights for lesbian and gay communities.

WAYNE MESSAM

Messam, 45, defeated a 16-year incumbent in 2015 to become the first black mayor of the Miami suburb of Miramar. He was re-elected in March. The son of Jamaican immigrants, he played on Florida State University’s 1993 national championship football team and then started a construction business with his wife. He has pledged to focus on reducing gun violence, mitigating climate change and reducing student loan debt and the cost of healthcare.

JOE SESTAK

The retired three-star U.S. Navy admiral and former congressman from Pennsylvania jumped into the race in June. Sestak, 67, highlighted his 31-year military career and said he was running to restore U.S. global leadership on challenges like climate change and China’s growing influence. Sestak said he had delayed his entry in the race to be with his daughter as she successfully fought a recurrence of brain cancer.

TOM STEYER

The billionaire environmentalist, a force in Democratic fundraising over the past decade, said in January he was focusing on his efforts to get Trump impeached and get Democrats elected to the U.S. Congress. Steyer, 62, reversed course in July, saying other Democrats had good ideas but “we won’t be able to get any of those done until we end the hostile corporate takeover of our democracy.”

THE REPUBLICANS

Trump is the clear favorite to win the Republican nomination, and there has been criticism among his opponents that party leadership has worked to make it impossible for a challenger. Still, the incumbent will face at least two.

DONALD TRUMP

Serving in his first term, the 73-year-old real estate mogul shocked the political establishment in 2016 when he successfully secured the Republican nomination and then won the White House. His raucous political rallies and prolific use of Twitter were credited with helping him secure victory. After running as an outsider, Trump is now focusing his message on the strong economy, while continuing the anti-immigration rhetoric that characterized his first campaign as he vies for re-election.

JOE WALSH

A former congressman, Joe Walsh, 57, has become a vocal critic of Trump, who he argues is not a conservative and is “unfit” for public office. Walsh won a House of Representatives seat from Illinois as a candidate of the Republican Party’s fiscally conservative Tea Party movement in 2010, but was defeated by Democrat Tammy Duckworth in his 2012 re-election bid. After leaving Congress, he became a Chicago-area radio talk show host.

BILL WELD

The former Massachusetts governor, 74, ran unsuccessfully for vice president in 2016 as a Libertarian. He has been a persistent critic of Trump, saying when he launched his 2020 campaign that “the American people are being ignored and our nation is suffering.”