BATON ROUGE, La. — Republicans completed their conquest of the Senate on Saturday when Republican Rep. Bill Cassidy easily defeated Democratic Sen. Mary Landrieu in a runoff election in Louisiana.

The Associated Press called the race for Cassidy after early returns showed him leading by a wide margin.

“This victory belongs to you,” Cassidy told his supporters in Baton Rouge. “The people of Louisiana voted for a government that serves us but does not tell us what to do.”

The result gives the GOP its ninth Senate-seat pickup in this year’s elections. Republicans will have 54 Senate seats next year, and Democrats will control 46.

Cassidy capitalized on Louisiana’s increasingly conservative tilt and Landrieu’s shared party affiliation with President Obama, who is widely disliked in Louisiana.

Landrieu thanked supporters at her election headquarters in New Orleans.

“The joy has been in the fight,” she said, recalling her battles to send federal money to Louisiana after the 2005 hurricanes and the 2010 BP oil spill. “It’s been a blessing. It’s been a fight worth waging.”

Landrieu, the only Democratic senator from the Deep South and the only remaining Democrat elected statewide in Louisiana, worked during her campaign to distance herself from Obama, noting that she had opposed the administration’s moratorium on deep-water drilling in the Gulf of Mexico after the 2010 BP oil spill, and supported construction of the Keystone XL pipeline.

Cassidy, meanwhile, repeatedly told voters Landrieu has voted in line with Obama’s positions 97% of the time, and reminded them that she voted in 2010 for the Affordable Care Act, which is unpopular in Louisiana.

That strategy seemed to be effective. An average of polling data compiled by RealClearPolitics had Cassidy ahead leading by 20 points heading into the runoff.

“All Bill Cassidy has to do is take a knee and let the time run out,” Joshua Stockley, political science professor at the University of Louisiana at Monroe, said ahead of the final vote count Saturday.

Jeffrey Sadow, a political scientist at Louisiana State University at Shreveport, said Landrieu’s vote for the 2010 health care law “basically kind of broke the dam” in terms of her ability to win election to a fourth term.

Sadow said both candidates did a good job with the hands they were dealt.

“It’s just that Cassidy had a much better hand to begin with, and he didn’t squander it,.” he said.

Cassidy’s obvious edge helped convince national Democratic Party committees that Landrieu wasn’t a solid investment and their money would be better used elsewhere. Landrieu was badly outspent in the weeks leading up to the runoff.

Landrieu, 59, focused on maximizing turnout among black voters, but the breakdown of people casting early ballots suggested they would not participate in large numbers.

The race was marked by negative attacks from each side. In the final days of the race, Landrieu accused Cassidy, a part-time teaching physician at Louisiana State University, of charging the school for hours that he never worked. Cassidy denied any wrongdoing.

Cassidy and his supporters attacked Landrieu for improperly spending taxpayer money on charter flights that included campaign stops. Landrieu apologized and said her campaign reimbursed the federal government for the flights.

With Landrieu’s hopes for re-election in serious peril, Democratic leaders in the Senate arranged to hold a vote on her legislation to approve the Keystone pipeline, hoping passage of the bill would boost her chances in the runoff. But the bill came one vote short of passing, a stinging defeat. A similar bill backed by Cassidy, meanwhile, passed in the House.

Landrieu’s campaign touted her chairmanship of the Senate Energy and Natural Resources Committee, but once it was clear the Nov. 4 elections had given Republicans control of the Senate in the next Congress, her committee post essentially became a non-issue.

Baton Rouge pollster Bernie Pinsonat said no polls had been conducted on the Senate race for some time because “you don’t spend money on polls when you know the outcome.”

“The bottom line is, it’s not about Cassidy,” Pinsonat said. “It’s about Landrieu and her support of Barack Obama. Barack Obama got 18% of the white vote in Louisiana when he won. Mary Landrieu got 18% of the white vote on Nov. 4.”

Contributing: Deborah Barfield Berry, USA TODAY