After months of campaigning, the Democratic Party primary will at last begin in February, with voting in the Iowa caucuses. And after the field of candidates swelled to nearly two dozen, it has continued to contract.

Still, ahead of next summer's Democratic National Convention in Wisconsin, a dozen-plus politicians are vying for voters, fundraising dollars and media attention against President Donald Trump.

But a small group stands out in the field, including former Vice President Joe Biden, who remains the likeliest person to face Trump in the 2020 election.

Among other prominent names past and present, such as Sen. Cory Booker and former Texas Rep. Beto O'Rourke (who dropped out in November), Biden and three others have pulled ahead thanks to their polling averages and the millions of dollars they've raised.

Here's what you need to know about them.

RELATED: Meet the 2020 Presidential Candidates

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Former Vice President Joe BidenNathan Congleton/NBC/NBCU
Joe Biden

National polling average as of this writing: 27 percent, according to RealClear Politics
Fundraising in the second quarter of 2019: $15.7 million

The vice president under Barack Obama, Biden emerged early as the presumed frontrunner in the race to the White House — and that's where he's stayed since launching his campaign in April, despite at-times insurgent challenges from others and relatively uninspiring fundraising.

The 77-year-old has been a major figure in the Democratic Party for decades, serving as a senator from Delaware from the early ‘70s until 2009, when he left to be Obama’s second-in-command. During his time in Congress, Biden served on both the Senate Foreign Relations Committee and the Senate Judiciary Committee.

Biden has said his priorities as president would be rebuilding the middle class, tackling climate change, reforming the criminal justice system and protecting the Affordable Care Act, a signature achievement in Obama's White House.

RELATED: Joe Biden's Son Hunter and Hunter's New Wife Joined the Family to Support Former VP at Democratic Debate

He has not been without controversy, given his long political career: This year he’s issued apologies for both how he handled Anita Hill’s sexual misconduct testimony against Supreme Court Justice Clarence Thomas while on the Senate Judiciary Committee and for working with segregationists decades ago.

He also denied claims in April that he’d acted inappropriately toward former Nevada politician Lucy Flores, who said Biden touched her shoulder and kissed the back of her neck without consent in 2014. However, he later acknowledged he had made multiple women "uncomfortable" and said he would be "more mindful about respecting personal space in the future."

Biden's personal life has seen its share of tragedy and triumph. A 1972 car accident killed wife Neilia and their daughter, Naomi, though sons Beau and Hunter survived. Biden married his second wife, Jill, in 1977, but lost Beau to brain cancer at age 46 in 2015. (Beau’s widow, Hallie, went on to date Hunter, though the couple later split and Hunter secretly married Melissa Cohen.)

The 4 Democratic Candidates You Should Know in the 2020 Presidential Race_2
Sen. Bernie SandersSteve Rogers Photography/Getty Images for SXSW
Bernie Sanders

National polling average as of this writing: 16 percent
Fundraising in the second quarter of 2019: $25.3 million

Sanders, a senator from Vermont since 2007, surprised many with his seemingly quixotic 2016 run for president — which then came closer than many predicted to besting Hillary Clinton for the Democratic nomination.

A democratic socialist born and raised in New York City, Sanders, 78, has long championed progressive policies that he’s helped push into the mainstream, such as "Medicare-for-all," a $15 minimum wage, tuition-free public universities, community colleges and trade schools, criminal justice reform and a broad push to address climate change.

RELATED: Feeling the Bern: Cardi B Sits Down with Bernie Sanders to Chat About the 'Future of America'

With Massachusetts Sen. Elizabeth Warren also running for president, Sanders is no longer the only leading progressive in the race, as the two are tackling many of same big issues. (Warren previously called for free tuition at two- and four-year public colleges, though her plan was subject to income, while Sanders’ has no eligibility limitations.)

Polling shows that, like Biden, Sanders faces voter concerns about his age.

Sanders, who would become the first Jewish president should he be elected, has been married to wife Jane O’Meara since 1988 and considers her three children his own. He also has a son, Levi, who last year made an unsuccessful congressional run, as well as seven grandchildren.

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Sen. Elizabeth WarrenJoe Raedle/Getty Images
Elizabeth Warren

National polling average as of this writing: 14 percent
Fundraising in the second quarter of 2019: $24.6 million

In 2013, Warren became the first woman to ever be elected to the Senate from Massachusetts, a victory after she served as chair of the Congressional Oversight Panel during the 2008 financial crisis known as the Great Recession. She also helped launch the Consumer Financial Protection Bureau.

A former Harvard University law school professor who specializes in bankruptcy law, and who has been noted for the breadth and detail of her suggested policies, the 70-year-old Oklahoma native supports progressive issues such as student loan debt cancelation and, like Sanders, government-backed universal health care via "Medicare-for-all."

She’s condemned policies described as privileging the wealthy and has proposed a wealth tax on the 75,000 richest people in the country in order to pay for some of her other proposals, including her student loan plan.

RELATED: Elizabeth Warren Turns Down a Fox News Town Hall, Labeling the Network 'Hate-for-Profit Machine'

A favorite target of Trump's, who insults her as "Pocahontas," Warren continues to grapple with controversy because she identified herself as Native American in the '80s and '90s. Last year Warren attempted to prove she indeed had Cherokee Nation ties with a DNA test — and though it confirmed she likely did have indigenous blood, she was criticized by prominent members of Cherokee Nation.

“Using a DNA test to claim any connection to the Cherokee Nation or any tribal nation is appropriate and wrong,” Cherokee Nation Secretary of State Chuck Hoskin Jr. said.

“I am sorry for harm I have caused," Warren said at a Native American forum earlier this month, according to The New York Times. "I have listened and I have learned a lot, and I am grateful for the many conversations that we’ve had together.”

She has been married to husband Bruce Mann for 38 years and has a son, daughter and three grandchildren.

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Mayor Pete ButtigiegSarah Rice/Getty
Pete Buttigieg

National polling average as of this writing: 11.4 percent
Fundraising in the second quarter of 2019: Raised $19.1 million

A military veteran more commonly known as “Mayor Pete,” Buttigieg would be the first openly gay president if elected.

Thanks in part to his rhetorical style and striking personal story, the 37 year-old mayor of South Bend, Indiana, went from essentially unknown to serious contender in only a few months. His platform — sometimes criticized as too light on specifics — includes support for universal health care and a system he calls “Medicare for All Who Want It,” debt-free college for lower-income families, support for the "Green New Deal" to address climate change and universal background checks during gun purchases.

RELATED: It's 'Almost Certain' We've Had Gay Presidents Before, Pete Buttigieg Says — but Who?

Buttigieg served as a lieutenant in the U.S. Navy Reserve and was deployed to Afghanistan, earning a Joint Service Commendation Medal for his counterterrorism work.

Married to husband Chasten last year, Buttigieg speaks seven foreign languages and would be the youngest U.S. president ever if elected.
Who Else to Keep Your Eye On

Many remain in the running alongside the four above — hoping their campaign skills will capture a larger share of voter interest.

Among those who could break out are Sens. Booker and Amy Klobuchar (currently polling at 1.8 and 2.4 percent, respectively).

Booker, a longtime New Jersey politician, announced his candidacy in February with an explicit call for unity, saying, "We’ve got to begin to see each other with a far more courageous empathy to understand that we have one destiny in America."

Klobuchar, of Minnesota, hopes to build a successful campaign on her Midwestern political style and can-do-it record focused, in her words, on problem solving and not partisanship.

Elsewhere in the race, more unorthodox candidates such as entrepreneur Andrew Yang (and long-shot spiritualist Marianne Williamson) are running along with former New York City Mayor Michael Bloomberg and former Massachusetts Gov. Deval Patrick.