Barack Obama won the presidency by exploiting a political environment that devoured George W. Bush in a second term plagued by sinking credibility, failed legislative battles, fractured world relations and revolts inside his own party.

Mr. Obama is now threatened by a similar toxic mix. The disastrous rollout of his health care law not only threatens the rest of his agenda but has also raised questions about his competence in the same way that the Bush administration’s botched response to Hurricane Katrina undermined any semblance of Republican efficiency.

But unlike Mr. Bush, who faced confrontational but occasionally cooperative Democrats, Mr. Obama has a Republican opposition that has refused to open the door to any legislative fixes to his health care law and has blocked him at virtually every other turn.