I doubt it. The people splitting off would be the pro-choice, Leftist Democrats. There aren’t enough of them to gain a 1/3+ plurality. The most likely winner in that scenario would be the Republican candidate.
Allow me to relate what happenned in Illinois when two allegedly “pro-life” candidates ran for governor about ten years ago.
The Republican, George Ryan, had once been a leader in the state legislature on many social conservative issues. So he was trusted by the establishment conservatives.
The Democrat, Glenn Poshard, was a solidly pro-life Congressman of outstanding moral character. He was running for governor, in part, because of a self imposed term limit as Congessman which he was honoring.
Poshard beat out the pro-choice candidates in the Democratic primary. He was also what you might consider conservative on many of the other moral issues, though (as a Democrat) more liberal on other social concerns.
The crafty Ryan, seeing this, positioned leftward. He claimed to be on board with what he always had stood for and shored up the support of his longtime allies in the process. At the same tme, he insisted that these people were not his base.
The most leftward of Democrats jumped ship to support Ryan, figuring that he would better suit their interests than Poshard. The hardline conservatives supported Ryan because he was their boy and, well, they were “Republicans”, afterall, who (in many cases) probably despised the idea of electing a Democrat, even if he seemed solid or could help move the overall situation of the state’s political climate in a positive direction with such a position of influence.
The party faithful loyalists on each side just voted their ticket, as usual.
In an extremely close race, Ryan won. He ended up disappointing the conservatives greatly (leaving them feeling betrayed), helped to send the state’s Republican Party to ruins, and is now heading off to the federal penitentiary (as do so many Illinois Governors, it seems.)
The point is that most people just vote the party ticket to which they are culturally or geographically affiliated with. People who have particular concerns which they don’t see getting addressed by the main candidates may jump ship. They can, however, have a significant influence if there are enough of them, particularly from varying areas of interest. Indeed, it might be suggested that the present Republican party is just such a thing, which has coalesed a wide swatch of groups to form a stronghold and majorities.
Given, it can be quite difficult for a third party candidate to win office. And, in the scenario above noted, you may be correct that the Republican would be the most likely winner. Though it is also possible that a third party candidate might come up with an agenda which would attract voters from both sides, siphoning off significantly dissatisfied citiizens, to the point where the unexpected could happen… such as the Democrat winning or even the factors which play into the established party electorates splitting votes and lowering turnout while the third party person rallies those with a passion enough to make a difference that he somehow slides through, or at least makes it into a runoff (if a state’s system works that way) where he could well win the ultimate election.