Bernie Sanders has been the Democratic frontrunner since jumping in the 2020 race. That's about to change.

Scott Audette/Reuters

Sen. Bernie Sanders listens to then-Vice President Joe Biden at the Democratic National Convention in Philadelphia, Pennsylvania, on July 27, 2016.

• Sen. Bernie Sanders' status as the early 2020 Democratic frontrunner could face its first serious challenge starting on Thursday, when former Vice President Joe Biden is poised to hop in the race.
• Biden has consistently been at the top of national polls and polling in early primary states, despite not officially declaring his candidacy.
• Sanders' campaign has gotten off to a flying start in multiple respects, especially fundraising, but evidence suggests Biden has the potential to undermine that momentum.

Over the past several weeks, Sen. Bernie Sanders of Vermont has emerged as the early frontrunner for the 2020 Democratic presidential nomination, but that could all change on Thursday when former Vice President Joe Biden is expected to hop in the race.

Sanders has dominated a large, diverse field of candidates up to this point, but through it all, former Vice President Joe Biden has been the elephant in the room.

Biden is expected to launch his 2020 campaign via an announcement video on Thursday, and the former vice president and senator from Delaware has the potential to pose the first serious challenge to Sanders' frontrunner status.

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While Biden's personal history and policy positions have already come under scrutiny prior to the impending announcement, he's still in a strong position to supplant Sanders as the frontrunner.

Biden has been at the top of the polls for weeks - even without officially declaring

The former vice president is dominating in national polls for the 2020 Democratic primary.

Biden is six points ahead of Sanders in the latest national polling from Morning Consult, placing him in the lead for the 2020 Democratic nomination. The poll showed 30% of Democratic primary voters support Biden, while 24% support Sanders. The closest contender, Mayor Pete Buttigieg of South Bend, Indiana, came in with 9%.

Additionally, Morning Consult has found that Sanders supporters would choose Biden over any other candidate if they had to make a second choice.

Morning Consult has also polled voters in early primary states, where Biden also has a commanding lead with 36% of voters in these states supporting him compared to 26% for Sanders.

Early primary states like Iowa and New Hampshire serve as barometers for how candidates will fare in the long-term. Candidates who perform poorly in these states typically drop out early.

While Sanders has pulled ahead of Biden in a few polls from early primary states, particularly New Hampshire, the data have shown Biden as the top choice for voters nationwide.

Biden and Sanders are the most recognizable 2020 candidates, but the former vice president still has higher favorability

While Biden and Sanders have the most name recognition of anyone in conversations surrounding 2020 Democrats, Biden does have a slight advantage when it comes to favorability.

According to an analysis from FiveThirtyEight, which looked at recent national, Iowa, and New Hampshire polls of favorability ratings with Democratic voters, Biden has the highest average favorability rating at 74% and Sanders comes in second with 70%. Meanwhile, Sen. Elizabeth Warren of Massachusetts comes in third with 60%.

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Sanders favorability is strong nationally, but he trails behind Biden in early primary states. If these numbers stay steady, it could spell trouble for the senator down the line.

When it comes down to it, no other declared candidate has presented a significant challenge to Sanders so far. But Biden has managed to play an outsized role in the race so far without formally declaring.

If the former vice president does finally announce on Thursday, and it seems likely that he will, Sanders will no longer be at the top of the polls among declared candidates.