Mr. Trump, who has been accused of stirring up racial strife, handed his critics more ammunition on Sunday when he refused the opportunity to distance himself from Mr. Duke and the Klan. Asked repeatedly to do so in an interview with CNN, he demurred. “You wouldn’t want me to condemn a group that I know nothing about,” he said. “I would have to look.”

Later he backtracked, posting on Twitter, “I disavow.”

Mr. Rubio’s headfirst lunge into a bout with Mr. Trump is a striking turnaround that the Florida senator himself calls disappointing. But it also reflects a conclusion that his above-the-fray approach was ineffective against a front-runner who seems to gain popularity with each fight he picks.

“I had hoped that this would be a campaign only about ideas,” Mr. Rubio told the crowd of more than 3,000 here, in the far suburbs of Washington, as he accused Mr. Trump of being a fraud, a threat to national security and possibly even a racist.

“I need your vote Tuesday,” Mr. Rubio told his audience, which was about as rowdy and animated as any he had drawn. “Friends do not let friends vote for con artists.”

Virginia is one of more than 10 delegate-rich states that will vote in the Super Tuesday contests this week — others include Texas, Georgia, Alabama and Massachusetts — most of which Mr. Trump is favored to win.