LARGO, Md. — President Obama mounted a passionate, campaign-style defense of his health care program on Thursday, just days before its main elements take effect, mocking opponents for “crazy” arguments and accusing them of trying to “blackmail a president” to stop the law.

Addressing a friendly audience outside Washington, the president abandoned the professorial tone he sometimes takes while describing the program and departed from his text to fire up supporters. He portrayed critics as billionaires who would deny help for the sick, and politicians who have become hostage to Tea Party ideologues.

Mr. Obama, his voice laced with scorn, ridiculed Republicans for threatening to shut down the government or refusing to increase the debt ceiling to undercut the health care program, saying they had “put up every conceivable roadblock” and were “poisoning Obamacare” so they could then “claim it’s sick.” He cited some of their more flamboyant quotes in an attempt to portray them as extremists, including one racially charged comment.

“All this would be funny if it wasn’t so crazy,” Mr. Obama told hundreds of students, professors and others at Prince George’s Community College. “A lot of it is just hot air, a lot of it is just politics, I understand that. But now the Tea Party Republicans have taken it to a whole new level because they’re threatening either to shut down the government or shut down the entire economy by refusing to let America pay its bills for the first time in history unless I agree to gut a law that will help millions of people.”