DES MOINES — Bernie Sanders edged ahead of his Democratic rivals in Iowa, affirming his resurgence less than four weeks before next month’s caucuses, according to a new poll from The Des Moines Register and CNN.
The poll showed that Mr. Sanders was the first choice for 20 percent of would-be caucusgoers, an increase of five percentage points from November, when The Register last polled the state. He was followed closely by Elizabeth Warren at 17 percent, Pete Buttigieg at 16 percent and Joseph R. Biden Jr. at 15.
The results are the latest sign that Mr. Sanders — lifted by his loyal supporters and an unchanging message — has strong campaign momentum heading into the Feb. 3 Iowa caucuses and has rebounded politically after having a heart attack in October. It is the first time he has led a Register poll this cycle. In 2016, Mr. Sanders battled Hillary Clinton to a virtual tie in Iowa, transforming him into a threat for the Democratic nomination.
The poll was less kind to Mr. Buttigieg, who held a dominant lead in the last Register poll, with 25 percent support. That poll also showed Ms. Warren at 16 percent and Mr. Biden at 15 percent.
To say the poll was highly anticipated is an understatement. The poll is the first significant survey from Iowa in nearly two months, a drought that has left a murky picture of the Democratic primary race in a state that conducts its first-in-the-nation caucuses. Iowans famously break late, sometimes making their final decision in the weeks and days before the caucuses occur, and every campaign will be scrutinizing the results for signs that their candidate is strengthening — and that their rivals are weakening — heading into the final stretch.
The poll numbers are the latest evidence that the race in Iowa remains fluid and winnable for the top four candidates, who have all crisscrossed the state in recent weeks to try to persuade supporters to come out for them on caucus night. Only 40 percent of respondents said their minds were made up, a reflection of the indecision on the ground, where conversations with Iowans often reveal that they still favor multiple candidates.
Of paramount importance to many Democrats in Iowa is beating President Trump in the general election in November. But the absence of a clear front-runner, along with no indication that any one candidate will break away from the pack in the remaining weeks, has left many voters unsure where to align their preferences.
Amy Klobuchar, who has attracted more interest in recent weeks but has yet to convert that into an increase in actual support, held steady at 6 percent, good for a distant fifth place in the poll. Cory Booker, who is hoping for a lucky break, also remained unchanged at 3 percent, behind the entrepreneur Andrew Yang, who was at 5 percent.
The poll of 701 likely Democratic caucusgoers was conducted Jan. 2-8 and has a margin of error of plus or minus 3.7 percentage points.
Political officials here widely believe that there are five tickets out of Iowa this year, instead of the typical three.
Much has changed since The Register last released its poll on Nov. 16. Kamala Harris, who was once considered a top-tier candidate but had seen her standing severely slip, dropped out of the race, leaving her supporters scrambling to find an alternative candidate to back. The House of Representatives voted to impeach President Trump. And escalating tension with Iran has pushed foreign policy to the forefront of a primary race that had so far focused more squarely on domestic issues.
The deadline to qualify for Tuesday’s Democratic debate was 11:59 p.m. on Friday, and normally, an Iowa poll released hours before such a deadline could bolster candidates on the cusp of qualifying. But this time, no one was on the cusp.
Mr. Booker and Mr. Yang both met the Democratic National Committee’s donor requirement, but Mr. Yang had only one qualifying poll of the required four, and Mr. Booker had none. The Iowa poll gave Mr. Yang his second qualifying mark, but it was not enough to get him into the debate.
Maggie Astor contributed from New York.