DES MOINES — Sen. Elizabeth Warren surged in a respected poll of likely Iowa caucus-goers released on Saturday, testing former vice president Joe Biden’s dominance in the state.

The poll is the first from the Des Moines Register, together with CNN and Mediacom, to put the two-term Massachusetts Democrat at the front of the pack, with 22 percent saying she was their first choice for president. Biden, who led the Register’s June poll, sits at 20 percent, putting the two in a statistical tie, far ahead of the rest of the field. Sen. Bernie Sanders (I-Vt.) slipped 5 percentage points to a distant third, at 11 percent.

The September survey is the first by the Register in the 2020 cycle not to put Biden in the lead in the first-in-the-nation contest, which will take place in February. The closely watched results arrive as Warren builds momentum with immense crowds, including 20,000 in Manhattan earlier this week.

The poll was conducted Sept. 14-18 among 602 probable caucusgoers, with a margin of error of plus or minus 4 percentage points.

Filling out the top five were Pete Buttigieg, mayor of South Bend, Ind., and Sen. Kamala D. Harris (Calif.). Buttigieg, whose campaign had a commanding presence at Saturday’s Polk County Steak Fry, fell by 6 percentage points from the June poll.

Sens. Cory Booker (N.J.) and Amy Klobuchar (Minn.) were both at 3 percent, ahead of a cluster of candidates registering at 2 percent and eight others polling at 1 percent or less.

The results show considerable room for movement over the next half-year. Only 1 in 5 likely caucusgoers said they were set on their first-choice candidate, while 63 percent said they could envision supporting someone else.

Sixty-three percent also said it was more important to them that the winner of the Iowa caucuses could beat President Trump than that the victor in February shared their positions on major issues.

Warren has steadily gained ground in the Register’s polling, starting at 8 percent in December, before she had announced her candidacy. She came in at 9 percent in March and 15 percent in June.

The June poll differed from the survey whose results were released Saturday in seeking to take into account Iowa’s plan for a “virtual caucus,” which has since been scrapped because of concerns about whether phone technology could be secured against hacking.

A Monmouth University poll released last month suggested the addition of an option to participate over the phone, rather than attending an hours-long meeting in the biting cold, would have expanded support for Biden. The new poll reverted to the methodology of the first two Register surveys, which considered only in-person participation.

Jodi Stanfield, a volunteer for Warren in Des Moines, said she previously supported Sanders and admired both candidates, whose platforms she described as similarly ambitious on issues of health care and education. She now favors Warren because she can “actually get the work done,” said Stanfield, 51, who works at the YMCA of Greater Des Moines.