HENRICO, Va. -- The lone Virgil Goode campaign sign on a stretch of Virginia road was far outnumbered by placards promoting Mitt Romney.That Goode's sign was there at all in this pivotal state served as a reminder that plucky third-party candidates like the Constitution Party's nominee could muck up the works on Election Day. Libertarian Party candidate Gary Johnson is daring dissatisfied voters to "waste" their vote on him in the 48 states where he's on the ballot."I hope to rain on the party. And by that I mean the two parties," Johnson, a former New Mexico governor, said Saturday as he wrapped up his late swing through battleground Ohio after a college-town push in Colorado. "I hope to rain on it big time."Some polls have shown that Johnson and former congressman Goode, two not-long-ago Republicans, are primed to pull down more votes than the difference between President Barack Obama and Romney in crucial states such as Colorado, New Mexico, Ohio and Virginia. Experts caution, however, that the overall tightness of the race tends to work against third-party candidates: In the end, voters migrate back to the main nominees.Every presidential campaign brings talk of a credible alternative emerging to test the Democratic and Republican nominees. Aside from Texas billionaire Ross Perot's 1992 campaign, the phenomenon has seldom panned out in recent times.A much-hyped bid to field a bipartisan ticket fizzled when Americans Elect, a group that had secured ballot space around the country, retreated in May. The aim among the 2012 hopefuls seems more about roiling the system.Republicans in Goode's home state tried unsuccessfully to keep him off the ballot. Some say he could throw the race to Obama if the outcome depends on Virginia's 13 electoral votes and Romney narrowly loses.