NEW ORLEANS (Reuters) - Senator Mary Landrieu conceded defeat to Republican challenger Bill Cassidy in a run-off election on Saturday, a result expanding the Republican majority in the United States Senate at the expense of one of the chamber's last-remaining southern Democrats.

Mr Cassidy, a congressman whose victory swells the ranks of Republicans in the Senate to 54, ran a disciplined campaign focused on linking Ms Landrieu to President Barack Obama, who is deeply unpopular among whites in Louisiana.

Ms Landrieu was fighting to remain one of only two Democratic senators from the south-eastern U.S. - a party stronghold a generation ago - after losses last month by Democratic incumbents in Arkansas and North Carolina.

The top Democrat on the Senate Energy and Natural Resources Committee, Ms Landrieu campaigned on her clout, a message complicated by her party's loss of Senate control.

Her campaign was also weakened by her recent failed push to win Senate approval of the Keystone XL pipeline from Canada's oil sands to the US Gulf Coast.