Vice President Joe Biden talks with Sen. Kamala Harris, D-Calif., center, and her aunt, Dr. Sarala Gopalan, during swearing-in ceremony in the Capitol's Old Senate Chamber, January 03, 2016.

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.