Democratic presidential hopefuls Former Vice President Joe Biden (L) and Vermont Senator Bernie Sanders participate of the seventh Democratic primary debate of the 2020 presidential campaign season co-hosted by CNN and the Des Moines Register at the Drake University campus in Des Moines, Iowa on January 14, 2020. Robyn Beck | AFP | Getty Images

President Donald Trump has spent a lot more of his time lately worrying about Bernie Sanders and Mike Bloomberg. The latest 2020 Democratic presidential primary polls gives some hints why. Sanders, a Vermont senator and a leading contender for the nomination, has carved further into former Vice President Joe Biden's lead in national surveys, according to a RealClearPolitics average. At the same time, former New York City Mayor Bloomberg has funneled more than $200 million into ads and climbed toward the top tier of Democrats in U.S. polls. He stands tied with former South Bend, Indiana, Mayor Pete Buttigieg in the latest average of national surveys.

A competitive field of a dozen Democrats hoping to challenge Trump in November has taken clearer shape ahead of the first-in-the-nation Iowa caucuses on Feb. 3. White House hopefuls have hopped across the state as they look for an edge in the nominating contest that can inspire surges in both fundraising and support.

Meanwhile, four candidates — Sanders and Sens. Elizabeth Warren, D-Mass., Amy Klobuchar, D-Minn., and Michael Bennet, D-Colo. — find themselves off the trail during a critical time as the Senate holds Trump's impeachment trial. Polls leading up to Iowa have found a logjam at the top in the state, suggesting four candidates could compete for the most delegates.

Sanders got more good news in New Hampshire on Thursday, as a poll from radio station WBUR found he had 29% of support in the state's primary. The next closest candidate, Buttigieg, trailed at 17%. While the survey gave Sanders a bump in an average of New Hampshire polls, the contest appears tight with less than three weeks to go.

Biden still has an edge in Nevada surveys and a commanding lead in South Carolina polls. South Carolina awards the most delegates out of the first four nominating contests.

Bloomberg, a late entrant to the race, has effectively sat out the early states. He has focused on building staff in the 14 states that vote on March 3, or Super Tuesday, and inundating televisions with ads across the country. The contests include two of the largest delegate prizes in the primary: California and Texas.