Sen. Kay Hagan filed for a second term Monday in Raleigh, setting the stage for what is expected to be an intense battle for her seat in November.
Hagan, a Democrat, ousted incumbent Republican Elizabeth Dole in 2008 in a surprise as she rode Barack Obama's strong showing in North Carolina.
Dole's campaign was damaged in the waning days of the election by an ad that attacked Hagan for attending a fundraiser held at the home of man who advised an atheist political action committee, even though Hagan was active in her home Presbyterian church in Greensboro.
The 2008 election was an expensive one, with the Democratic Senatorial Campaign Committee spending more than $11 million for Hagan and the Republican Senate election committee spending $5.8 million for Dole, according to USA Today.
Republicans believe it's a seat they can regain in 2014, and the race is expected to get national attention from the GOP.
North Carolina House Speaker Thom Tillis is among the Republican contenders, who include Cary obstetrician Greg Brannon, Heather Grant, Ted Alexander, Charlotte pastor Mark Harris and Edward Kryn.
Brannon was found guilty of giving false or misleading information to investors of a technology company that is now defunct. He said he was "treated unfairly by the court" and will appeal.
Hagan has portrayed herself as a moderate, while the GOP has sought to tie her to Obama, especially on health care.
Hagan became the first Democrat to hold the seat since Jesse Helms first won election in 1972. Helms served for 30 years before retiring, and Dole won the seat.
Before Helms, the seat was held by Democrat B. Everett Jordan from 1958 to 1973.
Other notables to hold the seat include former University of North Carolina President Frank Porter Graham and conservative Raleigh attorney Willis Smith, who beat incumbent Graham in the epic 1950 election that was highlighted by intense racial overtones.
Another legendary politician to hold the seat was Furnifold Simmons of New Bern. Simmons was a dominant figure in North Carolina politics who served in the seat for 30 years until 1931.