Jeb Bush, a possible Republican presidential candidate in 2016, offered to help run a campaign to oust him. Bill O’Reilly, the Fox News titan, said he wanted “to beat him up.”

And at a black-tie dinner a few nights ago, a billionaire donor to the Republican Party warned that New York City risked becoming the “New Havana” under his stewardship.

National Republicans, alarmed by the rising influence of activist liberals in government and eager to paint the Democratic Party as captive to its left wing, seem to have settled on an unlikely new nemesis: Mayor Bill de Blasio.

It is a startling turn for a municipal leader who was virtually unknown, even in New York, just a year ago, but now finds himself on the same conservative dartboard as far better established figures like Senator Elizabeth Warren of Massachusetts and the occupant of the Oval Office.