If this happened it might actually be the death knell for the Republicans. I can't see democrats losing in this scenario.
Libertarian vote isn't republican leaning, like you seem to assume. Most polling in races suggests that libertarians actually gather equal support for both the D and R if he/she wasn't in the race.
It would be really interesting to see it broken down by state which party the Libertarian vote takes more votes from. My guess would be that the Libertarians take more votes from the Democrats on the coasts and more from the Republicans elsewhere, but who knows?
