BATON ROUGE (AP) — Democratic U.S. Sen. Mary Landrieu formally signed up Friday for her re-election bid, shortly after top GOP challenger John Kennedy also registered for the race on the final day of Louisiana’s candidate sign-up period.

Fall election qualifying ended with a twist. State Rep. Michael Jackson, a longtime Democrat, entered the race for the Baton Rouge-based 6th District congressional seat as an independent candidate, a move that will let him bypass the party primaries and could hurt freshman Democratic U.S. Rep. Don Cazayoux’s chances to keep the seat he won in a special election last spring.

Cazayoux is one of six Louisiana congressmen running for re-election, but the top of the state’s ballot belongs to the Senate race, which has shaped up as a two-person contest.

After filing their paperwork and paying their registration fees, Kennedy talked of the need for change in Congress, while Landrieu spoke of the experience she’s gained during her 12 years in office.

‘‘You can’t name me one single problem in the last 20 years that Congress has solved for moms and dads … If you want to change the United States Senate, you’re going to have to change the senator,’’ Kennedy said.

Kennedy, now the state treasurer, ran as a populist Democrat in the 2004 U.S. Senate race, also on a theme of change, but he lost to Republican David Vitter for the open seat.

Landrieu, a former state treasurer herself, has been considered vulnerable as she seeks a third term in office, and she’s faced tight races each time she’s won the job. She’s amassed a formidable war chest for this re-election campaign and touts her seniority among Louisiana’s mainly new congressional delegation.

‘‘We need to start from strength, not from scratch,’’ Landrieu said.

Both candidates have raised millions, with each campaign reporting more than $1.5 million taken in during the latest fundraising period alone.

Four lesser-known candidates also entered the Senate race, including a Libertarian computer programmer from Baton Rouge, a Republican landscape contractor from Baton Rouge and a waiter at Harrah’s Casino in New Orleans who signed up as an independent and said he once was homeless. Another candidate, an independent, wouldn’t speak to reporters.

Six of the state’s seven congressmen qualified for their races: U.S. Reps. Steve Scalise, R-Jefferson; William Jefferson, D-New Orleans; Charlie Melancon, D-Napoleonville; Rodney Alexander, R-Quitman; Charles Boustany, R-Lafayette; and Cazayoux, D-New Roads. The seventh seat is open because Republican U.S. Representative Jim McCrery is retiring after 20 years in office, and his 4th District position representing northwest Louisiana attracted nine candidates.

Cazayoux was hailed by national Democratic leaders earlier this year when he took the 6th District seat that had been held for decades by a Republican, after Richard Baker resigned from the job. But he faces battles from Republican state Sen. Bill Cassidy of Baton Rouge and from Jackson.

Jackson, who lost his bid for the post when he ran as a Democrat against Cazayoux, said he switched to an independent for the best chance of winning. He’ll avoid the closed party primaries, which he called ‘‘a tough process,’’ but he said he would caucus with Democrats in the U.S. House if elected.

‘‘We’ll stick close to our Democratic principles and values, but realistically, we believe this is an easier route,’’ Jackson said.

Jackson’s entrance into the race is a blow to Cazayoux because the black state representative is expected to peel away some of the black support that helped propel Cazayoux to win the seat. Pearson Cross, a University of Louisiana at Lafayette political scientist, called a Jackson candidacy ‘‘devastating to Cazayoux’s chances.’’

A spokesman for Cazayoux, Lewis Lowe, released an e-mail statement when questioned about Jackson: ‘‘Congressman Cazayoux looks forward to a spirited campaign in the fall and welcomes the other candidates to the race. He looks forward to having a healthy debate on the issues. In the meantime, Congressman Cazayoux will continue to work hard in Washington and continue to be a strong voice for the people of the 6th District.’’

Only one incumbent didn’t draw an opponent: Melancon, who won the southeast Louisiana’s 3rd District seat by fewer than 600 votes four years ago.

Meanwhile, Jefferson attracted an array of competitors as he battles a federal bribery indictment. Seven fellow Democrats are opposing him for the New Orleans-based 2nd District job, along with one candidate each from the Green, Libertarian and Republican parties.

The last Democrat to sign up was Orleans Parish School Board member Jimmy Fahrenholtz, who said he didn’t agree with analysts who predicted the large number of opponents could help Jefferson. ‘‘I think it’s going to split the vote enough that pretty much anybody can win,’’ Fahrenholtz said.

The first party primary is Sept. 6. Party runoffs, if needed, will be Oct. 4. The general election is Nov. 4.