Joe Biden and Kamala Harris are likely the top contenders for Barack Obama’s endorsement in 2020

A huge question of the 2020 campaign is who former President Barack Obama will endorse.

A primary field including both Sen. Kamala Harris, D-Calif., and former Vice President Joe Biden could make the choice especially tricky since they would likely be the top contenders for his backing.

Harris, a first-term senator who also served as California’s attorney general, declared her candidacy for president Monday, while Biden, himself a longtime former senator, has yet to decide whether to launch a campaign.

Harris and Biden have had long and fruitful relationships with Obama. The former two-term president would arguably end up the biggest supporter of a Biden 2020 run, but Obama also counts Harris as a loyal ally. Harris was San Francisco district attorney when she threw her support behind Obama’s presidential run in early 2007 — a time when many other elected politicians in the nation’s most populous state backed then-front-runner Hillary Clinton.

With other Obama political allies likely to enter the 2020 presidential race, though, many Democratic strategists believe the former president won’t rush into endorsing anyone.

“Smart people and smart money — donors — are going to let this thing play out a little bit to see who is real and then decide where they want to push all their eggs or their chips into that candidate,” said Andrew Acosta, a California-based Democratic political strategist.

Other veteran Democrats, including some who worked on Obama campaigns, believe the former president won’t endorse a particular 2020 contender until the primary process is further along. Obama held secret meetings with Biden, Sen. Elizabeth Warren of Massachusetts and at least seven other presidential hopefuls last year, Politico reported in June.

An Obama spokesman didn’t respond to a request for comment for this story. CNBC also reached out to the Harris campaign.

Harris has been considering the presidential run for some time and has joined a growing field that already includes fellow Sens. Warren and Kirsten Gillibrand of New York.

Democrat Julian Castro, a former mayor of San Antonio who served as Obama’s secretary of Housing and Urban Development, also announced he’s running. Among other declared Democratic candidates are Rep. Tulsi Gabbard of Hawaii and John Delaney, a former congressman from Maryland. Pete Buttigieg, Democratic mayor of South Bend, Indiana, is officially exploring a run, too.

In all, there could be 20 or more Democratic presidential hopefuls entering the 2020 campaign, according to political strategists interviewed for this story.

Former 2016 presidential candidate Sen. Bernie Sanders, I-Vt. is said to be considering another run. Sen. Cory Booker, D-N.J., over the weekend visited the South and also is said to be close to announcing a decision. Los Angeles Mayor Eric Garcetti made a swing through Iowa last year and is said to be considering a run, too, as is billionaire former New York Mayor Mike Bloomberg. Democratic rising star Beto O’Rourke of Texas, who narrowly lost to Republican Ted Cruz in last year’s Senate race, is also weighing whether he should jump into the campaign.

Biden officially remains undecided about whether he will run. But polls show him as the top preference of Democratic voters, ahead of Sanders.

“Obama has a lot of loyalty to Biden, they are very close,” said Robert Shrum, director of the University of Southern California’s Dornsife’s Center for the Political Future and a former Democratic political operative.

According to Shrum, who served on multiple presidential campaigns, “Obama could obviously have a very large impact on this process if he wanted to, but I don’t know whether he will.”

Shrum recalled how Sen. Ted Kennedy of Massachusetts had a big role in the Democratic presidential race in early 2008 when he formally endorsed Obama right in the middle of primaries.

Meantime, Harris plans to have campaign headquarters in Baltimore and a second office in Oakland, California, where she grew up.

Even before she announced her presidential run, Harris generated headlines for her tough questioning of several Trump administration appointees. The former prosecutor has been seen as a rising star in the Democratic Party and is the daughter of immigrants from India and Jamaica.

She became one of Obama’s California campaign chairs in the 2008 presidential contest and gained a reputation as a strong fundraiser.

“She always got a strong reception from people when she spoke,” said Kim Mack, a former regional field organizer in California for the Obama 2008 presidential campaign. Mack isn’t currently associated with the Harris campaign.

If Biden decides to join the 2020 race, Mack believes it could dim the chances for Harris to win the Democratic presidential nomination. She said there’s a chance Harris could end up as a vice presidential pick, too.

“I really feel like it will be Biden-Harris,” she said. “And in order for that to be a successful ticket, she needs to raise her visibility across the country.”

Harris is scheduled to speak on Friday at an event at the South Carolina State Fairgrounds in Columbia. The Palmetto State could prove crucial for her candidacy as it is scheduled to be one of the first states to have its primary in February 2020 and follow New Hampshire.

In addition, the Iowa caucuses take place in early February 2020 and will be followed by the Nevada caucus later that month.

California’s role in the Democratic nomination is seen as more important in 2020. The state’s primary will be held along several other state primaries on Super Tuesday — March 3, 2020 – instead of its usual June spot, which is when the primary season traditionally starts to wrap up.

“California has been irrelevant for the most part in the Democratic process since 1972 when it was decisive,” Shrum said.

The veteran political watcher was quick to add, though, that California moving up the primary still doesn’t make Iowa and New Hampshire less important, and arguably makes them more important. He said that’s because those early states will help determine “who’s winnowed in, who is seen as a viable candidate.”

Also, Shrum said Harris may have the advantage of name recognition in her home state, but that doesn’t mean she’s guaranteed to win California.

“I don’t agree with the idea that somehow or another people would just vote for someone because they were from California, if they had not established their viability in the earlier part of the process,” Shrum said. He said online fundraising and social media strategies as well as whether a message resonates with a wide variety of people are among many factors that will help determine the candidates that do well.

Shrum recalled that former California Gov. Jerry Brown came in third place in the 1980 Democratic presidential primary in California, trailing Ted Kennedy and President Jimmy Carter. To be fair, he said Brown ran toward the end of his first stint as governor when he “wasn’t all that popular.”

“People don’t throw away their votes, and I think she [Harris] understands this. That’s why she’s talking about having a headquarters in the East and not just the West,” Shrum said. “And I would expect to see her compete vigorously in the early states.”