Over the weekend, the president condemned the shooting, calling it an anti-Semitic act and urging the country to come together to root out hate. He said he would tone down his speech in its wake, prompting boos from a rally crowd on Saturday night in Illinois. But that did not last long.

By Sunday afternoon, Mr. Trump was attacking Tom Steyer, one of the Democrats targeted with a mail bomb, and that night, he was blaming the news media for the poisonous level of discourse. On Monday, he charged in a Twitter post that news organizations incite “anger and outrage” with biased reporting.

The president also used the same language adopted by the suspect in the synagogue attack to condemn a caravan of Central American migrants making their way north through Mexico.

“This is an invasion of our Country and our Military is waiting for you!” Mr. Trump tweeted, referring to the migrants. The suspect in Pittsburgh posted a message on social media about the caravan shortly before the massacre, accusing Jews of bringing in “invaders” who were killing his people.

Later Monday, in an interview that aired on Fox News, Mr. Trump rejected the notion that he should have maintained a more somber tone after the carnage in Pittsburgh, saying that he was playing to a crowd that craved the spectacle of him savaging his political opponents.

“Rallies are meant to be fun, rallies are meant to be everything, and I said tone it down, and then you saw the group saying, ‘No, don’t tone it down! Don’t tone it down!’ So we had a great rally,” Mr. Trump said on “The Ingraham Angle.”

“Frankly, I think that’s probably the way it should be,” he said. “You should go about your life.”

Mr. Trump said he had nothing to do with helping to spread the kinds of messages that motivated the mail bomb-plot suspect, whose van was littered with pro-Trump messages and stickers attacking the news media and political figures the president has targeted. “He was insane a long time before — you look at his medical records,” the president said.