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Melinda Deslatte and Bill Barrow, The Associated Press Published Saturday, December 6, 2014 10:05PM EST

BATON ROUGE, La. -- Republican Rep. Bill Cassidy has defeated Democratic Sen. Mary Landrieu, denying her a fourth term and extending the GOP's domination of the 2014 midterm elections that put Republicans in charge of Capitol Hill for the final two years of President Barack Obama's tenure.

With Cassidy's victory, the GOP will hold 54 seats when the Senate convenes in January, nine more than they have now. Republican victories in two Louisiana House districts Saturday - including the seat Cassidy now holds - ensure at least 246 seats, compared to 188 for Democrats, the largest GOP advantage since the Truman administration after World War II. An Arizona recount leaves one race still outstanding.

In Louisiana, early returns showed Cassidy with a wide lead.

Landrieu had narrowly led a Nov. 4 primary ballot that included eight candidates from all parties. But at 42 percent, she fell well below her marks in previous races, leaving the incumbent scrambling in a one-month runoff campaign that Republicans dominated via the air waves while national Democrats financially abandoned her effort.

Landrieu's defeat is a blow for one of Louisiana's most famous political families, leaving her brother, New Orleans Mayor Mitch Landrieu, to carry the banner. The GOP sweep also denied former Gov. Edwin Edwards a political comeback; the colorful 87-year-old politician, who had served four terms as governor, sought to regain public office after serving eight years in federal prison on corruption charges.

In the South, Democrats will be left without a single governor or U.S. senator across nine states stretching from the Carolinas to Texas. And the House delegations from the same region are divided almost entirely by race, with white Republicans representing majority-white districts, while majority non-white districts are represented by black or Hispanic Democrats.

The Louisiana Senate race mirrored contests in other states Obama lost in 2012, with Landrieu, 59, joining Alaska Sen. Mark Begich, North Carolina Sen. Kay Hagan and Arkansas Sen. Mark Pryor in defeat. Democrats ceded seats in Montana, South Dakota and West Virginia after incumbents opted not to run again.

Like victorious Republicans in those races, Cassidy, 57, made his bid against Landrieu as more about Obama than about his own vision for the job. An Illinois native, Cassidy made few public appearances during the runoff, seeking to avoid missteps that could change the race.

But in a state where 73 percent of white voters on Nov. 4 told pollsters they "strongly disapproved" of the president, that was enough to prevent Landrieu from finding her footing. Cassidy also enjoyed a prodigious advertising advantage in the runoff: Of every dollar spent by outside groups during the one-month runoff, 97 cents benefited the congressman.

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