Victorious Republican Bill Cassidy: He focused his campaign on attacking Barack Obama.

Baton Rouge: Mary Landrieu, the last deep south Democrat in the US Senate, was handed a crushing defeat in a mid-term run-off election here on Saturday by Republican congressman Bill Cassidy, completing a generation-long shift towards the Republican Party taking political control of America's deep south.

Mrs Landrieu's 12-point loss in Louisiana means the Democrats will now be left without a single US senator or governor across nine states stretching from the Carolinas to Texas, confirming a shift that began in the late 1960s.

The loss by Mrs Landrieu, the incumbent Democrat and scion of a local political dynasty, means that Republicans will hold 54 seats in the 100-member US Senate when it is sworn in in January.

Bill Cassidy, the 57-year-old Louisiana congressman who defeated Mrs Landrieu, described the victories as the "exclamation point" on a 2014 midterm election campaign that racked up huge Republican gains and left President Barack Obama increasingly isolated in the White House.

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Dr Cassidy focused his campaign on attacking Mr Obama, whose personal approval rating has slumped to 45 per cent. Dr Cassidy's win marks a high point of Republican control in the South, which was once a stronghold of Democrats who had supported slavery and segregation, but whose standing was undermined with white voters by John F. Kennedy's Civil Rights Act of 1964.

With Dr Cassidy's victory in what had been the last undecided Senate race of the midterm elections, the Republicans gained a total of nine Senate seats, giving them 54 senators and firm control of the upper chamber when the 114th Congress convenes in January.

For Democrats, Saturday's outcome was yet another sobering reminder of their party's declining prospects in the South, a region they dominated for much of the 20th century. Dr Cassidy will join a fellow Louisiana Republican, David Vitter, in the Senate, making it the first time in 138 years that a Democrat from the state has not sat in the Senate.

Speaking to supporters at the Crowne Plaza Hotel here, Dr Cassidy said: "This victory happened because people in Louisiana voted for a government which serves us but does not tell us what to do. Thank y'all."

Even though Mrs Landrieu narrowly edged out Dr Cassidy in a primary in November, his victory was widely expected. A second conservative candidate with a significant following, Rob Maness, ran a strong third in the primary, and subsequently endorsed Dr Cassidy.

As in much of the South, Louisiana has seen many white Democrats defect to the Republican Party. Compounding Democrats' problems was Hurricane Katrina in 2005, which forced roughly 125,000 reliably Democratic voters to permanently move to other states.

Dr Cassidy is an associate professor of medicine at Louisiana State University who joined the House of Representatives in 2009. He closely followed the Republicans' overall strategy this year of nationalising congressional races and linking Democrats to Mr Obama.

New York Times; Telegraph, London