The Louisiana U.S. Senate race will be decided in a runoff, television networks projected on Tuesday, in a contest seen as critical to the Democrats' bid to keep control of the Senate.
Democratic Senator Mary Landrieu, 58, a third-term incumbent, and Republican U.S. Representative Bill Cassidy will face off again on Dec. 6 after neither won more than 50 percent of Tuesday's vote, the networks projected.
Landrieu, the only Democrat in Louisiana currently holding statewide-elected office, faces a difficult path to re-election, analysts have said.
Cassidy, a medical doctor, has sought to link Landrieu to President Barack Obama, who is deeply unpopular among whites in the state, citing in particular her support of the Affordable Care Act, or Obamacare.
Landrieu's victory six years ago was helped by strong turnout by black voters in a presidential election year. Analysts said they were unlikely to vote in such high numbers in the runoff.
Hurricane Katrina, which demonstrated Landrieu's ability to steer large recovery funds to her state, is also no longer a fresh memory for many voters.
With Republican Rob Maness, a retired U.S. Air Force colonel backed by the conservative Tea Party movement, now eliminated from the race, his backers could opt for Cassidy over Landrieu, who has campaigned on her clout as chairwoman of the Senate Energy and Natural Resources Committee.