WASHINGTON (AP) — Outside political groups are signaling they'll spend big money on political ads in a Louisiana Senate runoff, asking broadcasters there to set aside time for political ads to air after Election Day.

Senate Democrats' campaign committee has booked $1.8 million in air time for after Nov. 4. Its GOP rival has booked $2.8 million, according to documents obtained by The Associated Press.

Under Louisiana's political system, if no one wins a majority of the votes on the first ballot, voters choose between the top two candidates.

Democratic Sen. Mary Landrieu faces two Republicans: Rep. Bill Cassidy and tea party favorite Rob Maness. Most observers expect Landrieu and Cassidy to face each other in a head-to-head contest on Dec. 6.