Former Vice President Joe Biden Joe BidenBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Joe Biden should enact critical government reforms if he wins MORE leads the Democratic field in a new national poll, followed by Sen. Bernie Sanders Bernie SandersTrump, Biden will not shake hands at first debate due to COVID-19 Sanders tells Maher 'there will be a number of plans' to remove Trump if he loses Sirota reacts to report of harassment, doxing by Harris supporters MORE (I-Vt.), but all of the top-tier candidates had a slide in support.

Biden was supported by 19 percent of Democrats and Democratic-leaning independents, while Sanders was supported by 14 percent in the Reuters–Ipsos survey published Thursday.

They were trailed by Sen. Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.), with 9 percent support, South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice MORE (D), with 6 percent, and former New York City Mayor Michael Bloomberg Michael BloombergBloomberg rolls out M ad buy to boost Biden in Florida Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Bloomberg pays fines for 32,000 felons in Florida so they can vote MORE, who made his debut in the poll with 4 percent.

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The top four all saw a dip in support since the last Reuters–Ipsos poll. Biden's support slipped by 2 points, Sanders's support fell by 3 points, Warren's support declined by 2 points and support for Buttigieg fell by 1 point.

Those numbers are also significantly lower than those in the RealClearPolitics national average of polls. In that average, Biden has 27.8 percent support, Sanders 15.6 percent, Warren 14.2 percent and Buttigieg comes in fourth at 11.4 percent.

Nearly one-third of those surveyed in the Reuters poll, 31 percent, said they still don't know which candidate to support, the highest indecision level since the same poll in April.

Researchers surveyed 719 adults this week who identify as Democrats, independents and politically unaffiliated. It has a credibility interval of plus or minus 4 percentage points.

The Iowa caucuses, the first 2020 primary contest, will be held in less than two months on Feb. 3.

More than a dozen people are vying for the Democratic presidential nomination.