The poll is the latest evidence that the Democratic race in Iowa — and for that matter nationally — has narrowed to four main candidates and then everyone else. Even as contenders such as Kamala Harris have consolidated resources in the first-in-the-nation caucus state, they’ve failed to gain ground. Harris notched just 3 percent in the poll.

The numbers reflect the improbable rise of Buttigieg, a small city mayor from Indiana performing beyond expectations in Iowa, a state with a long history of making or breaking presidential candidates. Buttiegieg had lagged in polling even as he built national star power with standout fundraising that’s rivaled Warren and Sanders. The new poll firmly establishes him as a top-tier contender.

Stuck in single digits for months, Buttigieg broke out with a standout performance at a marquee Democratic Party dinner earlier this month. He’s drawn steadily larger crowds at the same time he has rapidly invested in the state, doubling his staff presence and opening 20 offices across Iowa in the past six weeks.

Buttigieg has also spent significant sums in recent weeks on digital and TV ads in Iowa, and has gone on two highly-publicized bus tours.

His rise comes at a pivotal time in the race, after two new candidates threaten to clog up the centrist lane that Buttigieg now occupies. Former Massachusetts Gov. Deval Patrick announced his candidacy this week, and billionaire Michael Bloomberg has filed in two states and pledged to spend $100 million nationally on anti-Trump ads.

In September’s Register poll, it was Warren who showed momentum, pulling ahead of Biden for the first time, 22 percent to 20 percent. Sanders had dropped to 11 percent. Warren has come under scrutiny lately over her embrace of Medicare For All, raising questions about her viability as a general election opponent. Still, the Massachusetts senator is widely viewed to have the best field organization of the field.