(CNN) When Sen. Bernie Sanders launched his first presidential campaign, he was going it alone -- the only candidate in a small 2016 contest offering a loud and clear contrast with Hillary Clinton, who had effectively cleared the field before the primary began.

Among the potential progressive alternatives to stand down that year, Sen. Elizabeth Warren was the one with the largest following. She had only been in the Senate since 2013, but her record of fighting corporate influence in Washington helped make her the target of a "draft" campaign launched by liberal groups. Warren's decision not to run helped elevate Sanders and set the stage for a fierce primary that would transform him into a frontrunner in his second presidential bid and one of the Democratic Party's most influential figures.

This time around, though, the story is different. Warren is firmly in the fight -- and her ascent has given rise to complex questions over what the progressive coalition looks like and how much overlap there actually is between the candidates' respective camps.

The 2020 primary has more than 20 candidates and, with the exception of Vice President Joe Biden, all the top tier hopefuls sound a lot more like Sanders than Clinton. That, as Sanders often points out on the stump, is a testament to the success of his last campaign and the force of his progressive message.

But there is no 2020 contender with whom he shares a greater affinity -- both politically and personally -- than Warren. Their non-aggression pact has held throughout the early months of the campaign, with both routinely pivoting away from opportunities to draw sharp distinctions between their candidacies.

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