Never in U.S. history has a third-party candidate been elected president.

“History suggests that the prospects are in fact pretty bleak,” said Julia Azari, a political science professor at Milwaukee's Marquette University. “The idea of a centrist majority that actually votes is most likely a myth.”

Yet that hasn't stopped the prospect of an independent candidacy becoming a recurring sotto voce whisper in the 2016 presidential race. First Republican front-runner Donald Trump threatened to deploy that option if he was treated unfairly by his party. And now former New York City Mayor Michael Bloomberg—the founder and majority owner of Bloomberg LP, the parent company of Bloomberg News—is considering a bid for the White House as a centrist, according to a person familiar with his plans who was not authorized to speak on the record.

That person said that any decision would depend, in part, on whether more ideological candidates win the major party primaries. Douglas Schoen, Michael Bloomberg's pollster, told the Washington Post on Tuesday that it is a “unique moment in the country's history” that would allow an independent candidate to run. Michael Bloomberg's personal spokesman, Marc LaVorgna, declined to comment. 

One huge challenge to any third-party candidate is that the two-party system consolidates power and resources in a way that outsiders of all stripes have been unable to break through. Quirks in the system give the nominees of the two major parties an enormous advantage in the race to 270 electoral votes.

Since the election of Abraham Lincoln in 1860 established a Republican-Democrat duopoly, third-party candidates have competed in some two dozen presidential elections, and all suffered crushing defeats. Just nine of them topped 5 percent of the popular vote. The eight most recent third-party attempts—including Ralph Nader in 2000 and Ross Perot in 1996 and 1992—failed to win a single electoral vote.

“The historical record has examples that run the gamut from the pre-Civil War Republicans who emerged from third-party status to become a major party replacing the Whigs, to countless third-party or independent candidates who have registered but a blip on the electoral screen,” said Walter Stone, a political science professor at the University of California at Davis.

In the last century, no third-party candidate has topped 19 percent of the popular vote and all have come in a distant third place. The last such candidate to secure any electoral votes was segregationist George Wallace in 1968, who won five Southern states as he fought against the civil rights movement. Arguably the strongest third-party candidate was Progressive Theodore Roosevelt in 1912, who won six states en route to a landslide defeat at the hands of Democrat Woodrow Wilson.

In a shift, the more recent third-party candidates over the last century ran in part to make a statement against the two-party system, in contrast to candidates like Wallace and centrist Republican John Anderson in 1980, who wanted to bolster ideologically losing factions within their own party.

“An important characteristic of most of the 20th century candidates in this vein is that they had belonged to major parties and broke away over policy and potentially also their own personal ambitions,” Azari said. “This is different from candidates like Perot in 1992, Nader in 2000, or the general idea of something like Americans Elect, who specifically want to offer an alternative to the two parties.”

Perot won 19 percent of the popular vote, besting Wallace's 14 percent, and yet he failed to win even one state. “Without regional appeal, it's more common for even relatively successful independent candidates, such as Ross Perot, to win a portion of the popular vote but no electoral votes, due to the winner-take-all nature of the Electoral College,” said John Sides, a political science professor at George Washington University in Washington.

Independent candidacies have typically been inspired by a yawning gap between the views of mainstream voters and those of either major party's nominee. The challenge, according to political scientists, is party polarization can have the counter-intuitive tendency to work against a candidate trying to occupy the middle, given that his or her supporters would likely prefer one of the major party candidates to the other.

“Bloomberg would face a dilemma embedded in a polarized party system,” said Stone, who co-wrote the book Three's a Crowd about Perot and the dynamics of third-party bids. “The temptation to run is based in part on frustration with partisan polarization, but partisan polarization makes it ultimately more difficult for an independent candidate to win votes in a winner-take-all system.”

As in the case of Perot, a challenge for Bloomberg is that his supporters are “likely to see their votes as helping their least favored candidate win,” said Stone. “When the parties are polarized, the least-favored candidate is much less palatable than when the parties are not polarized. This puts enormous pressure on Democrats to stick with their nominee and Republicans to stick with theirs.”

When Trump threatened a third-party run, the overwhelming consensus of political analysts and strategists, reflected in polls, was that he would split the Republican vote and help Democrats win the election. Similarly, Perot and Nader were both accused of playing “spoiler”—siphoning votes from one party and therefore helping the other win—for the Democratic and Republican parties, respectively.

While the challenges would remain enormous, Bloomberg could potentially bring a pair of strengths that are novel, Stone said: “Perot ran as a kind of centrist but had no governing experience, while Anderson was a centrist Republican loser in the 1980 nomination race who lacked the financial resources of Perot or Bloomberg.”

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