Democratic Sen. Mary Landrieu heads into a Dec. 6 runoff as a clear underdog, struggling for another six-year term against a wave of conservatism and Republicans looking to pad their new Senate majority.

Fifty-eight percent of voters chose another candidate on Tuesday over the 18-year incumbent. She has the distinction of being the last Democratic statewide elected official in a state where President Barack Obama remains highly unpopular. And her main campaign theme of clout was undercut when her party was forced into minority status: Even if she’s re-elected, Landrieu will lose her vaunted chairmanship of the Senate’s energy committee to a Republican.

And with that national question answered — Republicans will control the Senate with at least 52 of 100 seats, whether Landrieu wins or loses — the urgency of her quest for campaign cash and new voters has drained away.

Landrieu insists she can defeat Republican Rep. Bill Cassidy in the head-to-head matchup, despite consistent polling that shows her running behind in the runoff contest. She’s strung together victories in other runoffs. GOP leaders say they aren’t taking victory for granted, and Democrats say Cassidy has vulnerabilities Landrieu can exploit.

At a Wednesday event in New Orleans, Landrieu characterized the race so far as having been aligned against her because of the “national atmosphere” of frustration with Obama, from whom she distanced herself in many campaign speeches. Now, she said, voters in Louisiana will focus on the differences between her and Cassidy, and on what each can do for the state.

“I am encouraged, really, to be still standing in a night that was very difficult for the Democratic Party,” she said.

Immediate criticism came from some quarters of the Democratic base.

In New Orleans, state Sen. J.P. Morrell said the Landrieu campaign and the Democratic Party in general did a “lousy job of engaging African-Americans and keeping them engaged.”

“Mary’s got to realize that her success in Louisiana has always and will always depend on whether she can energize the African-American vote,” Morrell said, “and showing how much you disagree with the president doesn’t do it.”

Fundraising solicitations quickly went out from both campaigns, with Landrieu at a disadvantage. Cassidy, both political parties and outside groups booked millions of dollars in TV air time for the runoff before it was a certainty — locking in lower rates — but Landrieu’s campaign staunchly refused to reserve the space.

The Koch-backed Freedom Partners Action Fund rushed to the airwaves on Wednesday, starting a more than $2 million blitz of anti-Landrieu ads. The Karl Rove-backed American Crossroads network planned to keep throwing cash at the race, too, having spent $2 million ahead of Tuesday’s first round of voting.

“We’re determined to remain involved until Bill Cassidy is elected,” Crossroads spokesman Paul Lindsay said.

Senate Republicans’ campaign arm had booked $2.8 million in ads and Senate Democrats’ committee already had planned $1.8 million in ads. Neither had canceled their reservations.

While Cassidy skipped public events Wednesday, Landrieu unveiled a website highlighting votes she said Cassidy took against women, the elderly, students, veterans and disaster victims and kicked off a statewide tour to pan the congressman’s record.

Shifts in her campaign theme were evident.

On Wednesday, Landrieu switched from focusing on her chairmanship in energy-rich Louisiana to direct hits to Cassidy, calling him a “wishy-washy” politician who voted with his party’s leaders against his home state’s interests on such matters as the Social Security retirement age and disaster aid for his congressional district.

Cassidy wasn’t backing down.

“Sixty percent of the people in Louisiana have voted for change,” he said.

For his part, Cassidy will have to move beyond selling his candidacy as part of the GOP Senate takeover — any win by Cassidy would be irrelevant to Senate control. He’ll also need to coalesce support from voters who backed Republican candidate and tea party favorite Rob Maness in the primary.

Cassidy ran only 16,400 votes behind Landrieu among the crowded field of eight candidates. Maness carried more than 202,000 votes and hasn’t directly endorsed Cassidy.

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Associated Press writers Cain Burdeau in New Orleans, Bill Barrow in Atlanta and Philip Elliott in Washington contributed to this report.