"Runoffs have been required in TX special elections since 1958"
Are TX special elections all-party affairs? If so, it makes sense to require a run-off if no candidate gets 50%+1. But I don't see how such all-party election could withstand First Amendment scrutiny. If political parties have a First-Amendment right to have their members nominate a standard-bearer for their party, as was decided by the Supreme Court and the Ninth Circuit when they invalidated the Washington and California blanket primaries, having a special election in which several candidates from the party run and in which the top two among all the candidates go on to a run-off would not allow the members of the party to decide who the party's nominee will be (perhaps the Republican that finished in the top-two did so by getting votes from Democrats). Of course, the same holds true for the Louisiana November jungle primary (which is exactly like the TX special election), and it hasn't been found unconstitutional yet.