By Adam Wollner

McClatchy Washington Bureau

WASHINGTON, D.C. — The 2020 race is on.

Without a clear front-runner, Democrats are bracing for the possibility of their largest presidential field in recent memory. As many as 30 contenders, ranging from former candidates to current officeholders to political outsiders, are weighing a bid for the White House. However, many Democrats believe that ultimately only about half that many candidates will officially jump in into the race against President Donald Trump.

The prospective candidates have already spent much of the last two years checking off the traditional pre-presidential race boxes: publishing books, delivering speeches, traveling to early-voting states and wooing influential activists, donors and politicians. Rep. John Delaney of Maryland even skipped the usual exploratory phase and launched his long-shot campaign outright in the summer of 2017.

“It’s been overwhelming the number of potential candidates that have reached out,” said Gene Martin, who chairs the Manchester, N.H., Democratic Party.

Delaney won’t be alone for long. A handful of Democrats considering a run have said they will decide by the end of this year.

Here are the potential 2020 Democratic presidential contenders to watch.

FORMER CANDIDATES

Perhaps no contender will have a more outsized impact on the shape of the field than former Vice President Joe Biden. After spending the final stretch of the midterms barnstorming for fellow Democrats, Biden has said he will decide whether to run by year’s end.

Sen. Bernie Sanders, the runner-up in the last Democratic primary, appears inclined to run again, as does fellow 2016 candidate Martin O’Malley, the former Maryland governor who finished a distant third.

And John Kerry, the former secretary of state and 2004 Democratic presidential nominee, has not ruled out another bid.

FORMER OBAMA CABINET OFFICIALS

At least two other former cabinet officials from President Barack Obama’s administration are openly considering a run.

Julian Castro, the former Secretary of housing and urban development and San Antonio mayor, isn’t playing coy, saying he is “likely” to run. And former Attorney General Eric Holder, who currently leads a redistricting-focused group, has said he will make his intentions known by early 2019.

SENATORS

The largest subset of the 2020 Democratic presidential field could come from the U.S. Senate. Sens. Cory Booker, Kirsten Gillibrand, Kamala Harris, Amy Klobuchar, Jeff Merkley, Elizabeth Warren and Sanders have all taken many of the usual steps towards a bid.

Sens. Sherrod Brown and Chris Murphy are often mentioned in the presidential discussion as well, but both have shot down speculation they are considering a run.

Only three sitting senators have gone directly from the upper chamber to the Oval Office: Warren Harding, John F. Kennedy and Barack Obama.

GOVERNORS

Aside from O’Malley, a number of current and former governors are testing the presidential waters. Former Montana Gov. Steve Bullock, outgoing Colorado Gov. John Hickenlooper, Washington Gov. Jay Inslee, former Virginia Gov. Terry McAuliffe and former Massachusetts Gov. Deval Patrick have all visited early nominating states over the past year.

New York Gov. Andrew Cuomo harbors presidential ambitions, but he ruled out a bid earlier this year, pledging to fully serve out his next four-year term.

MEMBERS OF THE HOUSE

The odds of getting elected president from the House aren’t great. President James Garfield was the only one to accomplish that.

Delaney, who is leaving Congress after three terms, is already a candidate, while Reps. Tulsi Gabbard, Seth Moulton, Tim Ryan and Eric Swalwell are publicly considering joining him.

MAYORS

Over the past decade, Democrats have struggled mightily to develop their bench at the state level. But mayors have been one bright spot.

South Bend, Ind., Mayor Pete Buttigieg, New York City Mayor Bill de Blasio, Los Angeles Mayor Eric Garcetti and former New Orleans Mayor Mitch Landrieu have all made clear they are interested in seeking the White House.

History would be against them. No one has ever gone directly from the mayor’s office to the Oval Office.

THE BILLIONAIRES

Democrats could also go the route Republicans did in 2016: nominate a wealthy businessperson.

The Democrats’ two biggest spenders during the 2018 midterms, former New York City Mayor Michael Bloomberg and San Francisco hedge fund manager Tom Steyer, are actively considering converting from donor to candidate.

And with a nationwide book tour starting early next year, former Starbucks CEO Howard Schultz has stirred up speculation that he may run for president as well.

THE TRUMP AGITATOR

Finally, there’s attorney Michael Avenatti, who has risen to prominence representing porn star Stormy Daniels in her lawsuit against Trump. He’s become a fixture in the media, making the rounds to early states and assembling a team of political advisers. Avenatti has said he will make a decision on the race by the new year.