Is early voting constitutional?

My friend Steve brought up a great point, at least as far as food for thought goes:

I was just reviewing the Constitution for a lesson this week, and It occurred to me. Is Early voting Constitutional? The Constitution specifically sets the election day as the "First Tuesday after the first monday of November". The Constitution also gives the state the power to set up and run the election.

Lets make this clear, I'm not one who believes any votes should be thrown out because of this. I also believe every Citizen of the USA should have the right to vote, even Felons and other undesirable people of unrespectable standing. If you're an American, you should be able to vote.and the Vote should count whether or not it's for a Republican, Democrat, Libertarian, Socialist, Communist or Nazi. However, I also agree the Constitution is the supreme law of the land, and for good or bad, it's the law we live by in the USA. This could be an issue with the post election debacle to come over what votes get counted or not.

While I disagree with the "right" of felons to vote (another topic for another day, and valid arguments on both sides exist), I do admit that I've thought about the constitutionality of early voting. It's a valid question. My position (and I predict the courts would rule this way, if they haven't done so already):

A court will rule (to avoid throwing out potentially millions of votes) that early voting is permitted, but that LATE voting is not, i.e. AFTER Election Day. Because the argument could be extended to exclude absentee ballots, and courts have allowed those since the beginning of time, the courts would therefore permit early ballots.

Footnote on voting after Election Day (and that's voting, not counting of votes such as overseas votes that come in after Election Day but were cast before):

Recall that in Election 2000, the folks in Palm Beach County who were too dumb to read the butterfly ballot. They sued to get a new election in the county. The court rejected it on two constitutional bases: (1) a federal or state election cannot be held for one county only; and (2) the Constitution specifically states that the Election Day is held on the "First Tuesday after the first monday of November." Argument #2 would seem to exclude absentee ballots (and thus early ballots), but absentee ballots have always been counted (and allowed by the courts), thus early ballots would likely be counted. The court tuled in the Palm Beach County suit, though, that Election Day had come and gone, thus no late (i.e. post Election Day) voting would be allowed.

I'm not a lawyer, so only time (and maybe lawsuits by the losing party) will tell if this holds true or not. Let's hope we don't have to find out the hard way like we did four years ago.