Republican Party leaders fueled the shift to the right by promising results to their conservative base that they could not deliver: banning abortion, eliminating the deficit, slashing federal regulations, cracking down on L.G.B.T.Q. rights and greatly cutting taxes. Mr. Gingrich’s “Contract With America” — and the government shutdown it caused — set the stage for decades of unkept promises and primed primary voters to turn against the moderate and liberal elected officials who had once been a critical component of the Republican coalition, especially in the Northeast, when those officials could be readily blamed for not sufficiently supporting the party line.

As Republican voters and nominees adopted an increasingly extreme agenda, even a Republican Congress could not produce the results they had promised. While Republican officials delivered significant tax cuts for the very wealthy and, under George W. Bush, put numerous conservatives onto the federal bench, they failed to meaningfully relieve the tax burden for working- and middle-class people or to fully realize any of their culture-war goals, instead seeing same-sex marriage become the law of the land.

These failures drove a further rightward shift that resulted in the rise of the Tea Party. And when the Tea Partyers failed to stop President Barack Obama and his Affordable Care Act, we arrived at the 2016 presidential primaries and the rise of Donald Trump. The base of the party had become angry and alienated because the Gingrich-era promises had not been delivered.

During this period, some Republicans in the Northeast swam against the tide. Senator Jim Jeffords of Vermont, my predecessor in the U.S. House, became an independent before his retirement. In her last term, Senator Snowe cast a critical vote in committee to put the Affordable Care Act before the full Senate. She believed health care reform merited consideration by the full Senate, not a quiet death in committee. She favored “governing” over “controlling.”

But even in New England, long a bastion of liberal and moderate Republicanism, moderates are now losing in Republican primaries. This year, a Trump-backed candidate won the nomination for governor of Massachusetts; candidates endorsed by Donald Trump or who deny the validity of the 2020 election won races in New Hampshire; and Vermont Republicans nominated a right-wing figure for Senate. Increasingly, moderate candidates without a deeply established electoral history are unable to win nomination for major offices.

There have been a few moderate and liberal Republican success stories, but they are anomalies, peculiar to the person or the situation. In Vermont, Jim Douglas, governor from 2003 to 2011, and the current governor, Phil Scott, built long electoral careers and personal brands that made them more resistant to hard-right primary challengers. Gov. Charlie Baker of Massachusetts developed a reputation as a competent administrator in the 1990s, long before he ran for office.