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.