Sen. Bernie Sanders has gained ground in the Democratic presidential race and is now tied with former Vice President Joe Biden among the party’s primary voters nationally, a new Wall Street Journal/NBC News poll has found.

Mr. Sanders was the first choice of 27% of Democratic primary voters, essentially tied with Mr. Biden, who had 26%. Mr. Biden led in all previous Journal/NBC News polls of the 2020 primary race.

Sen. Elizabeth Warren slipped to a distant third place, with 15% support. In the prior two surveys, she was about even with Mr. Sanders for second place. Mr. Sanders has gained support by drawing voters from Ms. Warren, the poll found. (See the poll results.)

The results suggest a substantial reordering of the race in the days before Democrats kick off the presidential nominating process with Monday’s caucuses in Iowa, where several recent surveys show Mr. Sanders vying with Mr. Biden for the lead.

The survey also finds that Michael Bloomberg, the former New York mayor, has jumped into fourth place nationally on the strength of an ad barrage costing nearly $300 million so far, according to the political-ad tracker Kantar/CMAG. Some 60% of Democratic primary voters have seen a Bloomberg ad, the new poll found, far more than have seen ads by any other candidate.