Democratic U.S. Sen. Mary Landrieu won re-election to a third term Tuesday against Republican opponent John Kennedy, overcoming Republican expectations that Landrieu's seat could be the sole Democratic Senate seat to flip to the GOP.

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Landrieu, 52, never one to win easily, held onto her seat with a far smaller margin of victory than the double-digit lead shown in independent polls. Instead, she picked up 52 percent of the vote, compared to 46 percent for Kennedy, the Louisiana state treasurer, with nearly all precincts reporting.

"This was a hard-fought victory," Landrieu said at her victory party.

The incumbent senator was deemed vulnerable after Hurricane Katrina scattered the population of her New Orleans base of support, but Landrieu capitalized on voter enthusiasm for Democrat Barack Obama and also picked up backing from moderate voters who supported Republican John McCain for the presidency.

"Two years ago, the pundits said, 'She's not going to be able to do it. She's not going to be able to win again' because there was what they called a 'Katrina effect' ... but the fact is there was an opposite effect. People came back, and we came together. We are building a better state and a stronger state," Landrieu said.

Kennedy was making his second bid for the seat, his first as a Republican. He congratulated Landrieu and pledged to work with her.

"I extended to her my commitment to work with her every single day to make Louisiana as good as it can be, to try to solve the problems that moms and dads worry about when they lie down at night and can't sleep," he said in Baton Rouge.

The race was one of the nation's most expensive Senate contests, according to the nonpartisan Center for Responsive Politics' Web site OpenSecrets.org. The candidates -- and third party groups supporting them -- launched repeated TV attack ads that accused both Landrieu and Kennedy of pay-for-play politics, incompetence and lies about their records and positions.

National Republicans poured millions of dollars into advertising to support Kennedy as history would indicate the seat could be taken from Landrieu. She won the open seat in 1996 by fewer than 5,800 votes and retained it six years later by topping her GOP opponent by 42,000 votes. This time, she won by more than 100,000 votes.

At a polling place in New Orleans, Robert "Bobby" Ramirez said he didn't like Landrieu much but couldn't vote for someone who changed parties in midterm. "That's reprehensible," he said. "John Kennedy -- I don't know what he is," Ramirez said.

Landrieu made Kennedy's party switch a major issue, labeling him an opportunist who changed his positions to try to gain a higher political post. Kennedy changed parties last year, after being wooed by then-White House political strategist Karl Rove and other Republican leaders.

Landrieu also has easily outraised Kennedy, $10 million to $6 million, according to finance reports.

Kennedy tied himself and Landrieu to the presidential race, saying his support of McCain was more closely in line with the views of the majority of Louisiana voters. He called Landrieu's endorsement of Obama too liberal for the state.

As Obama's poll numbers soared and McCain's fell, Kennedy changed his message slightly.

He touted his endorsement of popular Republican Louisiana Gov. Bobby Jindal and called himself a conservative "firewall" against a possible Democratic-controlled White House and Congress. "It's going to be an important to have a U.S. senator who'll stand up for Louisiana principles and Louisiana values and Louisiana beliefs," he said.

But Landrieu, member of a longtime Democratic political family from New Orleans, tossed aside attempts to link Louisiana's Senate race to the bid for the White House.

Instead, she focused on her moderate voting record. She said it showed she voted not with the party, but with her constituents. And she talked of the billions of dollars in hurricane recovery and coastal restoration money she's brought to the state.

"I didn't go to the Senate to represent a political party. I'm independent in my voting record," she said.

The Democratic senator picked up support from McCain voters.

Construction company equipment manager Ben Tucker, 45, of New Orleans said his presidential vote went to McCain, but he said he voted for Landrieu because watching part of a debate left him feeling that Kennedy was evasive.

"Kennedy didn't answer some of the questions directly," he said.

Patti Guidroz, 53, voted for McCain and Landrieu.

"Not that I like her. But I think she can do more for the state than the other fellow," said Guidroz, who teaches high school in Jefferson Parish. Experience, she said, was a big plus for both McCain and Landrieu.

Kirby Goidel, a Louisiana State University political science professor, said Landrieu's record after Hurricane Katrina struck in 2005 was a strong one on which to run for re-election.

"She's been aggressive in fighting for the state, and she's achieved measurable, quantitative outcomes that she can point to," he said.

However, Kennedy portrayed Landrieu as a liberal who supports higher taxes and wasteful spending and framed himself as a conservative who wants to end deficit-spending and clean up a broken political system in Congress.

Jeffrey Kline, 21, who described himself as moderate and "Republican, most of the time," voted for Kennedy. He was in the tiny minority voting for Ron Paul as president. He said Obama and McCain were too socialistic.

In 2004, Kennedy ran for Senate as a left-leaning Democrat who backed John Kerry for president, criticizing President Bush's tax cuts. He lost to Republican David Vitter. Kennedy says party affiliation was never important to him and talked of his fiscal conservatism over the years.

Landrieu, however, called him "one confused politician," and ran repeated ads describing his stances in previous races as a Democrat that sometimes contradicted his positions in this race.

"The problem with Kennedy isn't the switch," Goidel said. "It's sort of the sense that it's politically calculated, and the Landrieu campaign has certainly capitalized on that."