(Reuters) - The largest field of Democratic presidential candidates in the modern U.S. political era got a bit smaller on Thursday when U.S. Representative Tim Ryan dropped out of the race, after failing to gain traction in opinion polls.

FILE PHOTO: Democratic presidential candidate and former Vice President Joe Biden speaks in Scranton, Pennsylvania, U.S. October 23, 2019. REUTERS/Mark Makela

Ryan, 46, a moderate from a blue-collar district in northeastern Ohio, is the latest Democrat to end a bid for the party’s nomination to take on President Donald Trump, the likely Republican nominee, in the November 2020 election.

Eighteen Democrats are still vying for the nomination, and three Republicans are making a long-shot challenge to Trump’s renomination. The latest to enter the Republican race is former U.S. Representative Mark Sanford of South Carolina, who announced his bid on Sept. 8.

The diverse group of Democrats includes six U.S. senators. A record five women are running, as well as black, Hispanic, Asian 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 crowded field, while others are still trying to break through. (Here is a graphic here of the Republican and Democratic presidential fields.)

DEMOCRATIC TOP TIER

Here are the Democrats who are ranked in the top five in Reuters/Ipsos polling:

JOE BIDEN

Biden, the early front-runner in opinion polls among Democratic presidential contenders, waited until late April to enter the race, launching his bid with a direct swipe at Trump. Biden, 76, served eight years as President Barack Obama’s vice president and 36 years in the U.S. Senate. He stands at the center of the 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 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.

ELIZABETH WARREN

The 70-year-old U.S. 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 (CFPB) after the 2008 financial crisis. Her campaign has surged in recent weeks, tying or leading Biden in some polls. 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 an array of policy proposals on everything from breaking up big tech companies to implementing a “wealth tax” on the richest Americans. Warren has sworn off political fundraising events to back her campaign.

BERNIE SANDERS

The U.S. senator from Vermont lost the Democratic nomination in 2016 to Hillary Clinton but is trying again. For the 2020 race, Sanders, 78, is fighting to stand out in a field of progressives running on issues he brought into the Democratic Party mainstream four years ago.

Sanders suffered a heart attack while campaigning in Nevada in October, but there has been little impact so far on his support.

His proposals include free tuition at public colleges, a $15-an-hour minimum wage and universal healthcare. He benefits from strong name recognition and an unmatched network of small-dollar donors.

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.

KAMALA HARRIS

The first-term U.S. senator from California would make history as the first black woman to gain the nomination. Harris, 55, 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 marijuana legalization. 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 in June but has since seen her numbers drop back down.

TRYING TO BREAK THROUGH

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

Slideshow ( 19 images )

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 the Medicare for All proposal, which is based on the existing government-run Medicare program for Americans 65 and older, and has warned that automation is the biggest threat facing U.S. 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.

AMY KLOBUCHAR

The U.S. senator from Minnesota was the first moderate in the Democratic field vying to challenge Trump. Klobuchar, 59, gained national attention when she sparred with Brett Kavanaugh during his Supreme Court nomination hearings last year. On the campaign trail, the former prosecutor and corporate attorney has said she would improve on the Affordable Care Act, popularly known as Obamacare, by adding a public option, and is taking a hard stance against rising prescription drug prices.

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

MIRED AT BOTTOM OF POLLS

Several candidates are stuck at the bottom according to numerous polls, with few breakout moments so far.

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’s populist, anti-war approach has won her fans among the far left and the far right, and she recently engaged in a Twitter war with Hillary Clinton, whom she called the “personification of the rot” after Clinton suggested Gabbard was being groomed for a third-party run at the presidency. Gabbard, 38, slammed Trump for standing by Saudi Arabia after the murder of journalist Jamal Khashoggi.

JULIAN CASTRO

Obama’s secretary of housing and urban development would be the first Hispanic to win a major U.S. party’s presidential nomination. Castro, 45, 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. In the third Democratic debate on Sept. 12, Castro drew jeers from the audience for an attack on Biden that was perceived as questioning the former vice president’s memory as a way to draw attention to his age.

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.”

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.

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

Montana’s Democratic governor, 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 the Medicaid healthcare funding program for the poor, increase campaign finance disclosures, bolster pay equity for women and protect public lands.

MARIANNE WILLIAMSON

The 67-year-old 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 to be paid 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. Messam’s Twitter profile says he is still running for president, but federal records show he raised only $5 during the third quarter of 2019.

JOE SESTAK

The retired three-star 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.

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 three rivals.

DONALD TRUMP

Serving in his first term, the 73-year-old real estate mogul shocked the political establishment in 2016 when he 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, 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 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 74-year-old former Massachusetts governor 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.”

MARK SANFORD

The former South Carolina congressman and longtime Trump critic lost his seat in the House last year to a Trump supporter in the Republican nominating primary. Sanford, 59, served two terms as South Carolina governor from 2003 to 2011 and served in Congress from 1995 to 2001 and from 2013 to 2019. His term as governor was marked with scandal when he admitted to traveling to Argentina to meet his mistress.