“I am ready to work with President Trump on things like infrastructure. I happen to agree with him on the water rule,” she added, referring to a regulation that Mr. Trump is seeking to roll back. “These are just lower priorities for them, I guess.”

Rather than trying to bring Democrats to his side, Mr. Trump has instead waged a war of Twitter insults against lawmakers who oppose his agenda. He has picked fights with allies, proposed giant budget cuts to programs dear to many in his own party and inserted himself into the health care fight in ways that hurt congressional Republicans’ efforts, all under the cloud of a federal investigation into possible connections to Russian meddling in the election.

All this has undermined the notion, born just six months ago, that Mr. Trump’s surprising win had rewritten the political map, as Ronald Reagan did in 1980, in a way neither party could ignore. Confident that the political order is largely intact, Democrats have been emboldened to oppose his agenda, and Republicans, who adamantly refused to help Mr. Obama, are learning what turnabout feels like.