Bernie Sanders — without appearing to realize it — put his finger on why his campaign hasn't succeeded.

"Poor people don't vote," he said in an interview with Meet the Press on Sunday. "It's just a fact."

He's right. It is a fact, and it's an important truth of American politics. But he didn't appear to recognize the implications. The fact that poor people don't vote, including for Bernie Sanders, proves that his campaign's theory of winning hasn't worked for them. Politically speaking, Sanders has identified the importance of the fact that "poor people don't vote" — but he's not going to be the one to make them do it.

Poor people don't vote

Bernie Sanders is absolutely correct. Poor people don't vote.

Or rather, they don't vote at anywhere near the rates their richer peers do.

This chart is from the 2014 midterm elections, and the electorate in midterm elections is even older, whiter, and richer than the electorate in presidential elections. But the electorate in presidential elections is still older, whiter, and richer than America as a whole.

More importantly for Sanders's argument, American nonvoters are substantially to the left of American voters — and substantially closer to the policies that Sanders is advocating.

All the available evidence suggests that if more Americans voted, they'd elect more politicians who wanted to redistribute income and spend more on programs for the poor. Sean McElwee of Demos made this case in much more detail for Vox in 2014, so you should check it out if you're interested.

The point of the "political revolution" was that Sanders would change who was turning out to vote

The problem with Sanders saying he's losing because "poor people don't vote," though, is that this wasn't a sad truth that he and his campaign discovered over the last several weeks. It — or rather, the possibility of fixing it — was at the core of his entire theory of winning.

Sanders isn't just running on his policy agenda. He's running on the idea of a "political revolution" that will allow him to accomplish that agenda. The theory of the "political revolution" is that Americans are so eager for free college and Medicare for all that they will not only sweep Bernie Sanders to the White House if he's nominated, but will elect more, and more progressive, Democrats down-ballot will then vote to pass Sanders's agenda through Congress.

Among people who typically vote, these policies aren't that popular. The "political revolution" is only plausible if it's about changing the composition of the electorate: bringing new people to the polls who don't normally vote, even in presidential elections.

But on those grounds, the "political revolution" theory is quite plausible. As Vox's Dylan Matthews pointed out earlier this month, 30 percent of eligible voters aren't registered to vote, or aren't accurately listed in the voter databases that campaigns use. Those voters are basically ignored by candidates. And, just like the nonvoting population as a whole, they're more likely to be poor than voters are — and more likely to support liberal policies on government spending.

A candidate who can figure out how to reach out to that 30 percent of voters could actually make a political revolution happen — or, at least, bring the median American voter to the left.

Bernie Sanders isn't the candidate who can make the "political revolution" happen

It's hard to mobilize that 30 percent of could-be voters, though. And it's pretty clear, at this point, that Sanders hasn't pulled it off.

Sanders hasn't been pulling in remarkable numbers of first-time primary voters. His base looks a lot like the existing progressive wing of the Democratic Party — the people who voted for Howard Dean over John Kerry and Bill Bradley over Al Gore.

And while Sanders has won low-income voters in some states (like Massachusetts), he's often lost to Hillary Clinton among the poorest voters — even in states that Sanders won overall, like Michigan. In fact, Sanders's contention that "poor people don't vote" was in response to a question about why states with the most economic inequality have tended to vote for Hillary Clinton.

You can debate why the poorest voters aren't supporting Sanders wholeheartedly. But the fact of the matter is that they aren't. And winning over poor people who do vote seems like an important prerequisite to winning over poor people who don't.

The bottom line is that Bernie Sanders and his campaign have identified an intriguing electoral strategy, but it's becoming pretty clear they're not going to be the ones who prove whether or not it works.

This should be good news for Sanders's supporters: Just because it looks very unlikely that this particular candidate will win the nomination this particular time, that doesn't mean that a different candidate won't be able to succeed with a similar platform and strategy.

And if the Sanders supporters committed to the political revolution devote their energy after this election to figuring out how to reach the "poor people (who) don't vote," that could be a game changer.