Rep. Tim Ryan had just been sworn in to Congress when he first voted for Nancy Pelosi to be speaker of the House in 2003.

The Ohio Democrat voted for her again in the next seven elections, helping elevate her to the role and making her the first-ever woman to reach the House’s highest post in 2007, and sticking with her even after Democrats lost the chamber in 2011.

In his 16 years in Congress, Mr. Ryan has never known a Democratic leader other than Mrs. Pelosi. He says it’s long enough.

Despite overseeing Democrats’ surge back to the majority, Mrs. Pelosi is facing a serious challenge from within her own ranks from a rump group of lawmakers who say the party needs new blood at the top. Mr. Ryan, who’s one of Mrs. Pelosi’s chief antagonists, says the party needs to look elsewhere to reflect Rust Belt states critical to Democrats’ fortunes, and the black women who are the “backbone” of the Democratic vote.