U.S. Sen. Mark Pryor of Arkansas has filed to run for re-election, setting the stage for one of the most expensive and closely watched Senate races in the country this year.
The Democratic two-term lawmaker filed paperwork at the Capitol on Friday morning to seek a third term.
Pryor was first elected to the Senate in 2002 and won re-election without a Republican challenger in 2008.
Republican U.S. Rep. Tom Cotton filed paperwork earlier this week to challenge Pryor and is the only expected GOP candidate for the seat.
The race has been in full swing since Cotton announced his bid in August, with both candidates and outside groups spending millions and swapping attacks through television ads.
Pryor is the only Democrat in the state's congressional delegation.