After she announced that she was withdrawing from the Democratic presidential nominating contest, Sen. Elizabeth Warren (D-Mass.) took questions from reporters Thursday. Asked if she had any regrets, she said she didn’t.

“I was told at the beginning of this whole undertaking that there are two lanes, a progressive lane that Bernie Sanders is the incumbent for and a moderate lane that Joe Biden is the incumbent for, and there’s no room for anyone else in this,” she added. “I thought that wasn’t right, but evidently it was.”

It was a remarkable bit of forthrightness from a political candidate, an admission that her campaign faced constraints that she herself had rejected. It also appears to have been an accurate observation — with a caveat.

One of the reasons that the Democratic contest has been narrowed to a contest between Sen. Bernie Sanders (I-Vt.) and former vice president Joe Biden is that Sanders and Biden had core bases of support that other candidates didn’t. Sanders’s was inherited from his 2016 run and is, as Warren observed, centered largely on his progressive policy proposals. Biden’s was based on his relationship with black Democrats — a demographic group that is largely moderate. That was the secret sauce, really: that the two ideological lanes happened to overlap with core bases of support that were enormously useful in a crowded field of candidates.

That overlap made Warren’s efforts to offer a middle ground tricky, particularly because voters largely saw her as fitting more in Sanders’s lane: Polling from The Washington Post and ABC News shows that Warren consistently ran better than her overall support with liberal Democratic primary voters and worse among moderate ones. That was the same as Sanders, though Sanders did less badly with moderates. Biden consistently did as badly with liberals as Warren did with moderates.

Entrance and exit polling from the contests that have been held so far shows how Sanders stood in Warren’s way. Biden consistently did better with those who identified themselves to exit pollsters as moderates, though that varied by contest. (In South Carolina, for example, Biden earned a plurality of the support in every ideological group.) For Sanders and Warren, there was a much sharper drop-off as voters got less ideologically liberal.

Warren may have wanted to see horizontal lines on the graph above, a steady measure of support regardless of ideology, but that’s not what she got. What she got was a pattern of support that mirrored Sanders’s, just lower.

It’s fair to ask why it was consistently lower. Speaking to reporters Thursday, she suggested that gender played a role, which is certainly true to some extent. It’s also true, as she said, that Sanders had a sort of first-mover advantage with more-liberal voters, and further that he had fostered a deeply loyal sensibility among those voters. The lanes were important, but the Sanders-Biden race we see now is also a race between two candidates who had a base Warren couldn’t assemble.

What’s also true is that Warren ran a robust campaign, doing all of the things that have powered past candidates to their party’s nominations. The problem was that she was stuck in a lane, and there was a big Volkswagen bus with a Sanders bumper sticker on it ahead of her.