In a lawsuit filed in state district court in Baton Rouge, the four claim the Baton Rouge Democrat is constitutionally banned from seeking another Senate term.

At issue is the Louisiana constitutional provision voters approved in 1995, which prohibits a legislator from signing up to run for another term after he has been "elected to serve more than two-and-a-half terms."

The date when Fields officially became a senator after his 1997 election is the basis of the legal fight.

The voters say Fields took office Dec. 13, 1997 -- one month and one day more than two-and-a-half terms.

Fields points to a state law passed in 2006 that clarifies that a legislator doesn't officially take office until he is seated by his colleagues.

In Fields' case, that would be March 23, 1998, which his attorney, Brace Godfrey, said meets the legal test.

Fields blames the "term limits" challenge on one of his opponents: Baton Rouge lawyer Jason DeCuir, a fellow Democrat. The two are among six seeking the District 14 seat, which includes Southern University, downtown Baton Rouge, some Baton Rouge subdivisions and LSU.

Fields said he expects to win the lawsuit.

DeCuir denies any involvement.