WASHINGTON — A rebellious group of 16 Democrats went public on Monday with their opposition to electing Representative Nancy Pelosi speaker when the new Congress convenes in January, taking the first formal step in a bid to force a leadership shake-up that could change the face of their party.

It is also exposing significant divisions just as Democrats take the majority.

In a letter to Democratic colleagues, 11 lawmakers and four newly elected members of the House declared that “the time has come for new leadership,” and said they would vote accordingly both when their party meets next week for an internal round of secret balloting to choose leaders and in a floor vote in January. (Another signatory, Ben McAdams of Utah, remains locked in a tight race against Representative Mia Love.)

The letter made official what had until Monday been little more than a Democratic whisper campaign aimed at demonstrating that Ms. Pelosi, 78, would not have the votes to become speaker. Its organizers have hoped that threat would compel her to step aside or broker a compromise aimed at bringing fresher faces to the helm of their newly ascendant party.

[Read the letter here.]

But so far Ms. Pelosi, the first woman to serve as speaker, has shown no willingness to do either, expressing supreme confidence that she will reclaim her old post.