Bernie Sanders is one of the most electorally successful non-major party candidates in United States political history. And he said Friday that voting for a third-party candidate for president in 2016 would amount to a "protest vote."

"Before you cast a protest vote - because either Clinton or Trump will become president - think hard about it," Sanders said on MSNBC's "Morning Joe." "This is not a governor's race. It's not a state legislative race. This is the presidency of the United States."

Sanders, of course, ran for president as a Democrat this year, shedding decades of working outside the two-party system that dominates American politics. And this is hardly the first time he's shunned a more progressive third-party candidate in favor of electing a Democratic president. As Dave Weigel wrote last year, he introduced Green Party presidential candidate Ralph Nader at a Vermont event in 2000, but he endorsed Al Gore over George W. Bush and then publicly shunned Nader's repeat candidacy in 2004.

But it's one thing for an independent to support a major-party candidate out of pragmatism; it's another to label a ballot cast for a fellow non-major party candidate a "protest vote."

And it's one a previous version of Bernie Sanders probably would have disagreed with.

Sanders actually rose politically thanks to what some would call protest votes. He ran four times in the 1970s for U.S. Senate and governor as a Liberty Union Party candidate, taking 2 percent, then 1 percent, then 4 percent, then 6 percent. He eventually parlayed that into a successful independent bid for mayor of Burlington, Vt., in 1981.

By 1988, Sanders endorsed Jesse Jackson for president when Jackson was running as a Democrat. This, at the time, was seen as a pragmatic move for the socialist mayor. But it came with a catch: He really wanted Jackson to run as a third-partier or independent.

"My own preference would have been, and I would have rather have seen, Jesse Jackson run independently, third-party, outside of the Democratic Party," Sanders said at the time.

Sanders also said in the same news conference that the two major parties would never create the kind of real change that was needed.

"Essentially, it's my view that the leadership of the Democratic Party and the Republican Party are tied to big-money interests and that neither of these parties will ever represent the people in this country that are demanding the real changes that have to take place," he said.

By 1992 and 1996, when Sanders joined the U.S. House, he endorsed Bill Clinton - but only as a lesser-of-two-evils candidate.

"Without enthusiasm, I've decided to support Bill Clinton for president," he wrote in 1996. "Perhaps 'support' is too strong a word. I'm planning no press conferences to push his candidacy, and will do no campaigning for him. I will vote for him, and make that public."

Sanders's evolution on the two-party system is three decades in the making. And it continued Friday with his "protest vote" comment.