Nothing reveals more about politicians than the decisions they make — why they chose to do something, how they made it happen, what came of it. The Washington Post is exploring one key choice by each leading presidential candidate and explaining the insight it offers into the way he or she might operate in the White House.

But now that the time had come to put his name on the presidential ballot, Sanders had made a decision that reveals a less-celebrated dimension of his political identity: He is also a pragmatist who likes to win. And so the first step in the Sanders revolution was also its most conventional.

The cranky outsider became a Democrat.

“It was a hard decision because he’s prided himself on his independence,” said his longtime friend Huck Gutman, an English professor at the University of Vermont. “He struggled with the fact that as somebody who had criticized the Democratic Party for a lack of forward thinking and a lack of courage that it could be a tough slog in the Democratic primaries.”

The wisdom of the decision bore its first fruits in the Iowa caucuses, where Sanders, 74, came within less than half a percentage point of upsetting Hillary Clinton, the establishment candidate for the Democratic nomination. Polls show Sanders with a commanding lead over Clinton in New Hampshire, which holds its primary Tuesday.

All of which makes the reasons for Sanders’s decision seem obvious. Independent campaigns for president have never won. And Sanders, who declined to be interviewed for this article, has said often that he would never be a “spoiler,” the label applied to third-party presidential candidate Ralph Nader, who was cast into political oblivion after being blamed with costing Democrat Al Gore the 2000 election.

“I have no taste for symbolic campaigns,” Sanders wrote in his 1997 political autobiography, “Outsider in the House.”

But the reasons go deeper than that.

Over his political career, Sanders has inched ever closer to the Democratic Party, with a congressional voting record more Democrat than most Democrats. At this point, he explains his democratic-socialist views by referring not to his old hero Debs but to one of the most beloved Democrats of all, Franklin Delano Roosevelt.

“Here is a very simple fact of life,” said Garrison Nelson, a political scientist at the University of Vermont who has tracked Sanders for years. “Bernie is the only socialist candidate I’ve ever met who wants to win. That’s the key to Bernie. He is one of the most competitive people I’ve ever met — very competitive, very smart. Because people think he’s a wooly-headed crazy man, they don’t understand how cunning he is.”