NEW ORLEANS  Mere survival was never the issue. Those familiar with Louisiana and its Republican senator, David Vitter, knew he would survive the summer of 2007, after he showed up on the client list of a Washington prostitution ring and then refused to address the matter beyond admitting to a “very serious sin” at a brief news conference.

What came as a surprise to many here is how he became a strong early favorite going into his 2010 re-election race. That turnabout is largely due to one person. “Along comes Obama,” said Elliott Stonecipher, a political analyst and demographer based in Shreveport, “and it changed everything.”

It is difficult to overstate President Obama’s unpopularity in most of Louisiana. He lost handily to Senator John McCain here, picking up only 14 percent of the white vote (the state is roughly two-thirds white). His health care plan is unpopular. His cap-and-trade plan to reduce greenhouse gases, in a state so dependent on oil and gas, is anathema.

In fact, in the South, which largely voted against Mr. Obama, the anger at his policies has been palpable, as shown by Wednesday outburst of a South Carolina Republican congressman, Joe Wilson, during Mr. Obama’s address to Congress.