Democrats, after two years largely spent simply opposing everything President Trump advocated, are defining themselves lately in ways Republicans are seizing on to portray them as far outside the American mainstream.

Casting Democrats as a scary and radical force is giving a fractured Republican Party a common thrust at a time when Trump’s standing even within his own party has started to dip. And it is giving Democrats a bit of the heartburn that Republicans have been grappling with for more than two years.

“There is legitimate concern among Democrats about policy and rhetoric that comes out of the very far left,” said Ed Rendell, the former governor of Pennsylvania and former chairman of the Democratic National Committee. “Yes, they hurt. It gives Republicans fodder to continue this train of thought that Democrats have become a socialist party. . . . They pounce on anything someone in our party says and make it seem like it represents the whole party.”

Abortion, taxes and health care have long been among the most combustible political topics.