We know John McCain will be on the presidential ballot in November. It's also safe to assume he'll be up against Barack Obama or Hillary Clinton. In most states, Ralph Nader will probably be there, too, though his influence is likely to be limited.

The next question, though, is who else might appear on the ballot.

One need not look too hard to find Democrats unhappy about the prospect of Nader splitting the left and helping McCain, but it's worth keeping in mind that Republicans may have a couple of challenges of their own.

For example, Alan Keyes -- who, rumor has it, kinda sorta sought the Republican nomination this year -- is moving closer to another campaign outside the confines of the GOP.

Former Republican presidential candidate Alan Keyes announced Tuesday night that he has left the GOP and is considering joining the Constitution Party.... "They're considering me, I'm considering them," Keyes said in a conference call late Tuesday night. "We have so much in common that I find it hard to believe we won't be able to work out a common basis for working together."

For that matter, if Nader makes it a three-way race, and Keyes makes it a four-way race, there's also the prospect of Bob Barr making it a five-way race, running as the Libertarian Party's candidate.

Third parties rarely matter at the presidential level, and I suspect the McCain campaign isn't especially worried about Keyes or Barr. It's not even clear if either would be able to qualify for the ballot in every state.

But in a close contest, a percentage point here or there might matter. At a minimum, it's something to keep an eye on.