An early poll of Iowa Democratic voters looking for a challenger to unseat President Trump shows them torn between familiar candidates in their 70s and one lightly experienced newcomer who is 46.

Two old hands — Former Vice President Joseph R. Biden Jr. and Senator Bernie Sanders of Vermont — and the new contender, the departing Representative Beto O’Rourke of Texas, grabbed the top three spots in a poll of likely Iowa Democratic caucusgoers released Saturday.

The results reflect the debate among Democrats over whether to run against the chaos-causing Mr. Trump with a solid résumé built on experience in Washington, or by nominating a fresh face who stirs passions. A majority in the Iowa survey favored pragmatism: 54 percent said it was more important to nominate someone able to defeat Mr. Trump than someone who aligns with voters’ positions. Similarly, 49 percent preferred a seasoned candidate compared with 36 percent who want a new face.

The results, 14 months before Iowa’s first-in-the-nation caucuses in February 2020, echo other recent polls that show the power of name identification so early in the nominating process, when no top candidates have officially announced they are running. Little known before the midterms, Mr. O’Rourke triggered Beto-mania with a dynamic campaign presence and prolific fund-raising in his narrow loss of a Texas Senate race to Ted Cruz.