There are several reasons to oppose this:
It only applies to "major" candidates. This discriminates against "3rd Parties" and puts the state legislature and courts in the position of enforcing the "2-Party System."
In this time of stolen and questionable elections, this starts the government on the slippery slope of postponing elections. Today's "Reasonable Delay" may well lead to tomorrow's "Emergency Measures." The Bush Administration already has plans to postpone this election in case of Terrorist Attack. Remember, until now, no major election has ever been postponed. US elections even went on through the Civil War without problems.
If a candidate dies, a special election will be ordered, which will cost just as much as a "postponed" election.
Measure 31 also only applies if the ballots have already been printed, which only covers a few weeks before the election.
This should be called the John Ashcroft Memorial Ballot Measure after he lost to Mel Carnahan, a dead man, in 2000. Ashcroft would have been much less dangerous in the Senate than as Attorney General.