If a third party gets a substantial percentage of the votes, both political parties will move in that direction to try and capture those votes (e.g., Ross Perot and the balanced budget).
IMO the Libertarian party should focus on Congress rather than the Presidential campaigns. If they had a substantial part of Congress they could do more with a smaller part of the electorate than they can with a failed Presidential campaign. If they had ten percent of the seats in Congress, neither of the other parties could pass legislation without the approval of the Libertarians. That would go a long way to prevent government meddling in our private lives.
So who in either party has made a huge push for balanced budgets that wasn't already due to Perot?
Thought so.
Both parties response to the third-party spoilers is to close their eyes, cross their fingers, and pray 'those freaks' don't show up to spoil the party again in four years.
As others have said, use the primary process to turn the incumbant bums out. Case in point, the guy in Utah who had a nearly 99% "A rating" from whatever Conservative think-tank who rates such things, except he was for illegal amnesty. He got turned out with a 60/40 landslide in the primary.
I know it's a catch-22 but the corollary to "nothing changes if you vote third-party" is that the other party you really hate cleans up, laughing all the way to victory, and nothing changes either, or it gets worse.
This is where the "Let the Libertarians run for dog-catcher first" argument comes into play. If you want policy/platform changes, you need to get your third party that represents your ideals a significant block in the legislature FIRST, then both dominant parties will have to modify their legislation and policy to woo the other voting blocks to get majorities.
Constantly focusing on executive branch elections to debate the "third party catch-22" is not very productive. The biggest effects the next POTUS is going to have on America is foreign policy and Federal Judges. Otherwise, all he will be doing is signing or vetoing the bills that Congress sends him.
The loss of stature and weakness internationally we'll suffer from Obama, (I somewhat ascribe to the more pissed off the rest of the world is, the better we're doing theory...) and the ultra-liberal justices he'll appoint
who'll serve for decades will have a much longer lasting effect than just the four/eight year election cycle. Even if America, the Republicans, or whoever, "wake up" and start running your dream candidates, those
bad changes will not be something he/she will be able to undo in your lifetime. Legislation can be reversed within an election cycle. 30 years of a SCOTUS Justice, or a complete shift in the geopolitical power balance, not so easy...
Ultimately, protest votes for executive office are LAZY. Because you can go down with the ship, striking a moral pose, and wash your hands of what happens next, even when in a way it's partially your fault. Whereas trying to run your third-party candidates for legislative races, where local involvement is higher,
and maybe that lesser of two evils executive only gets bills that are ones you like, just starts sounding a bit too much like "work"...
So YES, you vote for the "lesser of two evils" in the Executive races, and you make your principled stand at the local/legislative level.