McDaniel, a state senator who eked out a victory over Cochran in the June 3 Republican primary, has refused to concede after losing the June 24 runoff by a 6,700-vote margin. He alleges that Cochran's successful effort to expand his voter base to include Democrats resulted in "thousands or irregularities in the voting process."
The lawsuit, filed by the conservative group True the Vote, names 13 voters who it says "double-voted" -- cast ballots in Mississippi's Democratic primary and then in the Republican runoff.
Mississippi state law allows voters of one party to participate in a runoff of another, so long as "he intends to support the nominations made in which he participates." In practice, that usually means someone who voted in a Democratic primary could not then turn around and vote in the Republican runoff, but it is very difficult to prove that a voter did not intend to follow through and vote for the Republican in a general election.
"Having noted irregularities in the voter polls, the individual plaintiffs now suspect that their lawful votes in the Republican primary runoff have been impermissibly diluted by unlawful ballots cast in violation of the prohibition on 'double-voting,'" the lawsuit says.
McDaniel's campaign has been fundraising in hopes of mounting its own lawsuit, but has voiced support for True the Vote's effort.
"True the Vote is concerned with maintaining the integrity of Mississippi's election process," McDaniel said in a statement. "The voters should be able to trust that the manner in which their elected officials are chosen is not compromised, and that the rule of law is adhered to. It is vital we be allowed to examine election data to make sure that happens."
McDaniel supporters also claim the Cochran campaign paid Democrats $15 per vote.
True the Vote sends volunteer monitors to polls with the stated goal of preventing voter fraud. It has been accused of voter intimidation and was subject to a congressional investigation opened by Rep. Elijah Cummings, D-Md., in 2012. Its president, Catherine Engelbrecht, responded by filing a formal ethics complaint against Cummings earlier this year.