Conservative candidates and their supporters view him as a rival. They have every incentive to publicize those of his views that the G.O.P. considers heretical. More moderate and establishment-friendly Republicans oppose his statements and views on immigration. They fear that his tone will alienate Hispanic voters, who are considered essential to the party’s general election chances now and in the future. Mr. Trump’s supporters will start taking cues from the uniform opinion of their fellow partisans.

Until today, the big question about Mr. Trump was when coverage would shift toward a serious examination of him. Journalists and campaigns have, understandably, been reluctant to treat him as a serious candidate. The Huffington Post, for instance, decided it would cover him in the entertainment section. But eventually, campaigns were bound to treat him as a threat to be neutralized, and journalists would decide he was a candidate who needed to be covered.

Today, Mr. Trump brought the shift upon himself. His comments were nothing less than an invitation for the rest of the Republican Party to begin their long-awaited offensive. So far, the Republican National Committee, Rick Perry, Bobby Jindal, Jeb Bush and Scott Walker have already criticized him for his comments.

After today, Republican commentators and campaigns will have far fewer reservations about attacking Mr. Trump. They will be dismissive of his candidacy, and they will probably diversify their attacks, expanding the onslaught to include his record of donating to Democrats and his continuing support for universal health care. Nearly all of the campaigns have incentives to pile on, and Mr. Trump — without a deep base of support and with few party allies — will struggle to hold on.

He will probably try to stoke support and coverage with more attention-grabbing remarks, though my hunch is that his act will have lost its novelty by the time the attacks begin to take their toll. Voters will be looking for more from him than the bombastic campaign he has offered so far. They will be looking for a serious presidential candidate, and they won’t find one.