Open primary maybe, but there should be a proviso for runoff that mandates voting in the primary to be eligible for the runoff vote.
Mississippi has an open primary, but if someone voted in the Democrat primary a few weeks ago they can’t turn around and vote in the Republican run-off. Thus, while it would be legal for a black Democrat who sat out the primaries (or who was convinced to cross over and vote for Cochran in the GOP primary) to turn out and vote on the GOP run-off, most of the politically active black Democrats (whom presumably voted in the Democrat primary) are legally prevented from voting in the Republican run-off. We must be vigilant that no one who voted in the Democrat primary is permitted to vote in the GOP run-off, but need to acknowledge that it is not illegal for a Democrat to urge Democrats who did not vote in the Democrat primary to vote on the GOP run-off. And we should definitely point out to Mississippi conservatives that the Democrats are seeking to invade the GOP primary to nominate Cochran and that to prevent it all conservatives must vote for Chris McDaniel.
As a longer-term solution, the MS legislature should mandate party registration and closed primaries in MS.