Across the nation, a backlash to the Trump agenda is underway, especially among angry suburban voters like Dr. Mutnick. The latest example surfaced earlier this month in a Pennsylvania congressional special election, where Conor Lamb, a Democrat, defeated his Republican opponent in a district outside Pittsburgh that Mr. Trump had won by 20 percentage points.

In New York, Democrats are mostly focused on unseating six Republican members of the congressional delegation. But for some, merely targeting Republicans is not enough; Mr. Klein’s group, the Independent Democratic Conference, is now fair game because it is viewed as an enabler to Republican rule — helping them control the State Senate even though Democrats hold a numerical majority.

The rising antipathy toward the I.D.C. has led to a tentative agreement to have the renegade group eventually return to the Democratic fold. The condition-laden deal has the blessing of Gov. Andrew M. Cuomo, a Democrat who has been criticized by some — most recently by his new primary opponent, Cynthia Nixon — for not using his influence to force a truce years earlier.

Nonetheless, residual anger over Mr. Trump’s victory, combined with the Democrats’ hopes of flipping Congress in the midterm elections this fall, have invigorated efforts to drive the I.D.C. senators out of office.

A coalition of 60 anti-Trump groups has endorsed five Democratic challengers who will run against members of the I.D.C. in the September primary. The Working Families Party, which often cross-endorses Democratic candidates, has come out in support of seven challengers.