Currently the two party system leaves a void for many voters and the are seeking alternatives. The Libertarian party serves a middle ground that is able to provide an outlet for voters that are still looking for fiscal responsibility and value social freedoms and personal choice. Because it takes this stand, the Libertarian Party is in a good position to become a powerful third party.
The Libertarian Party will only become a major party when Americans are fed up with bickering between Democrats and Republicans. At that time, Libertarians will replace one of the major parties in Congress. The only major Libertarian we've seen in the past four years has been Rep. Ron Paul of Texas. He's technically a Republican, but he is the closest thing to a regular Libertarian candidate Americans have seen in politics. No libertarians are in Congress and they rarely make waves in statewide races. Until there is an epic disaster that decimates either Democrats or Republicans, the Libertarian Party won't become a major third party.