MONTGOMERY, Ala. (AP) — Alabama has a new law that prohibits voters from switching their political party allegiance between a primary and subsequent runoff.

Alabama does not require primary voters to register with a political party. The crossover voting ban is an attempt to prevent voters of one political party from trying to meddle in another party’s runoff – although there is a dispute about how much that actually happens.

“If you vote in one party’s primary, you can’t switch to the other’s runoff,” state Sen. Tom Whatley, the sponsor of the bill.

Whatley said voters aren’t required to cast a primary ballot to be able to vote in the runoff.

Secretary of State John Merrill said the law will be in place for an anticipated September runoff in the high-profile race for the U.S. Senate seat previously held by U.S. Attorney General Jeff Sessions. The crowded field of candidates vying to replace Sessions has increased the likelihood that a runoff will be needed. The primary will be Aug. 15 and the runoff, if needed, will be Sept. 26.

Whatley, a Republican from Auburn, said he has sponsored the legislation for three years and did not bring it with the Senate race in mind.

Merrill said under the law, state poll workers will have access to electronic voting books to determine if someone is ineligible to vote in a runoff.

The proposal was backed by members of the GOP, which dominates state politics with Republicans holding all statewide offices.

Democrats have long had a party rule prohibiting Republicans from voting in their runoffs, although it is largely unenforceable without court action.

Although Democrats have that rule in their party, Alabama Democratic Party Chairwoman Nancy Worley said the matter should be left to the parties.

“The Legislature has its own rules and would be enraged if any political party tried to pass rules for it to follow,” Worley said.

It was a dispute over crossover voting three decades ago that rocked state politics and helped give rise to the state’s first Republican governor since Reconstruction.

Then-Democratic Attorney General Charlie Graddick won the 1986 primary but lost the nomination to Lt. Gov. Bill Baxley in the courts, where Graddick was found ineligible after wrongly encouraging GOP crossover voting.

Graddick ran as a write-in candidate. Republican Guy Hunt became the state’s first GOP governor in modern times as Democrats. The state has only elected one other Democratic governor since, Don Siegelman who was elected in 1998.