WASHINGTON — There is a 45-year-old Democrat from the Rust Belt who has hosted yoga sessions on the Capitol lawn, a 40-year-old Harvard-educated former Marine from Massachusetts who made recruiting and supporting veterans to run for Congress a pet cause, a few seasoned lawmakers who rarely buck their party, and a handful of newcomers who campaigned promising change in Washington.

The 16 House Democrats who signed a letter on Monday calling for new leadership of their party are the leading edge of what threatens to escalate into a full-blown intraparty fight for control of the new majority. The letter was an unmistakable message to Representative Nancy Pelosi of California that she may not have the votes to reclaim the speakership she lost in 2011.

Their ideological leanings and political profiles are diffuse — certainly not the Democratic equivalent of the Freedom Caucus, the group of ultraconservative Republicans that often acts as a thorn in the side of G.O.P. leadership. What the group shares is a determination to shake up the top echelons of the House Democratic Caucus, whose leaders have remained unchanged for more than a decade, with Ms. Pelosi, 78, at the helm; Representative Steny H. Hoyer of Maryland, 79, as the No. 2, and Representative James E. Clyburn of South Carolina, 78, as the third ranking.

So far, no lawmaker has been willing to come forward to challenge any of them directly, and the list of signatories to the letter is limited, representing fewer than 10 percent of House Democrats. In fact, their ranks may be shrinking as Ms. Pelosi pulls them to her one at a time. On Wednesday, one of their most senior members, Representative Brian Higgins from Buffalo, dropped out and endorsed Ms. Pelosi.