McCain is threatening to leave Obama on stage with an empty podium Friday night.
Call his bluff, Barack: invite another presidential candidate to take McCain's place. There are several choices, but I suggest Bob Barr or possibly Chuck Baldwin.
Would this be a big win for the Libertarians and the Constitution Party? Sure. It would be huge for them. But any gains they make will be at the expense of the Republican party.
Are there downsides for Obama? I don't think so.
The Libertarian platform will look extreme and unhinged. The subject of the debate is foreign policy. While the Libertarian party believes that we should withdraw from Iraq, Barr has not embraced that position. Barr is likely repeat the standard, discredited Republican talking points that McCain would toss out, but without McCain's "noun-verb-P.O.W." gravitas. Barr might also preach Libertarian isolationism, which will look — and is — dangerous in today's world.
There is some danger to Obama, lowering himself to appear on stage with the Mickey-Mouse Libertarian Party, but I believe that this stigma will be attached to John McCain, not Obama. Obama will be there to espouse his views and it will be McCain's fault that his foil will be an extremist ex-Republican from a third party. In fact, Barr may be seen as a kind of proxy for McCain and his trouncing in the debate will raise questions about why John McCain couldn't be bothered to show up and defend himself.
If Barr wins over a single voter, he will win that vote from McCain. Perhaps some left-leaning Libertarians will even swing to Obama.