Colorado Gov. Bill Ritter is denying the obvious:

Gov. Bill Ritter says he is not a candidate to be Barack Obama's running mate.

His comments during a radio interview Monday morning follow a Washington Post column that said Ritter would be a great choice for the Democratic presidential candidate.

Ritter said he hasn't heard anything from Obama and does not believe he is being considered.

Correction: Goof on my part. Ritter is a Democrat, so the comments below are, um, not exactly relevant to Ritter's comments.

But it does raise a point about cross-party tickets. It is very unlikely Obama will choose a Republican as his running mate. Of the variety of reasons, perhaps the largest is that he doesn't need to. This is a Democratic year and Obama doesn't have to worry about his base leaving him, unless of course he puts a Republican within a heartbeat of the presidency.

McCain is a different story. Somehow McCain needs to fulfill the electorate's desire for change, which is very difficult for someone of McCain's age and Beltway experience. One way is via his running mate. But it seems that choosing a Democrat would be the equivalent of a Hail Mary. But, as the polls indicate, McCain doesn't need a Hail Mary right now. For all that this is a Democratic year, he's doing pretty well so far.