The president said they had flirted for years with a strain of bare-knuckled partisanship that appealed to people who sought to delegitimize him by falsely asserting that he was born outside the United States and was plotting to take Americans’ guns and impose martial law.

Now, he said, they should be held to account for the consequences.

“They just stood by and said nothing, and their base actually began to believe this crazy stuff,” Mr. Obama said. “Donald Trump didn’t start this. He just did what he always does, which is slap his name on it, take credit for it and promote it.”