When Sen. Bernie Sanders said recently that if Joe Biden implements his policy agenda, the presumptive Democratic nominee could be "the most progressive president since FDR,” he was probably right. In fact, something extraordinary is happening: Biden is getting more progressive in substance, yet it has done nothing to change his image as a moderate.

The Trump campaign is utterly flummoxed by this state of affairs; in reality, they long ago gave up trying to claim that Biden is some kind of extremist, and now can only shout that he’ll be a "puppet of the radical left.” But that doesn’t seem to be convincing anyone either.

There are a couple of reasons — and they’re pretty clearly the product of a careful strategy on Biden’s part. But the continuing evolution of Biden is a fascinating story, and one most of the public is probably unaware of.

Take, for instance, the climate change plan Biden released this week. While he put out a previous climate agenda, this one went much further, pledging to achieve 100 percent clean energy by 2035, spend $2 trillion over just four years on green infrastructure (as opposed to the $1.7 trillion over 10 years he had previously proposed), and aggressively pursue environmental justice, to make up for decades of policy in which low-income and minority communities bore the brunt of pollution.

The average voter — who right now is paying attention to the presidential campaign on only the most superficial level — probably heard next to nothing about it. But the reaction from progressives and climate activists ranged somewhere between surprise and joy. As one co-founder of the Sunrise Movement tweeted, the plan is "a VERY BIG DEAL, and is a huge victory for the #GreenNewDeal movement.”

Biden had room to move on this issue because the public is eager for stronger action on climate; as Data for Progress has shown, the steps he is supporting are extremely popular. But it’s not just the particulars. Biden — who has always been a politician with centrist impulses and a radar that seeks out the median location within the Democratic Party so he can stand there — is able to move left on substance because he has done such a good job of portraying himself as a moderate.

To be clear, I’m not saying it was some kind of con. But two things have happened. First, we went through a primary that was widely seen, in part, as a contest of ideas, with Sanders (I-Vt.) and Sen. Elizabeth Warren (D-Mass.) on one side and Biden (and some others from the party’s more moderate wing) on the other. He won precisely because Democratic voters decided that, as a moderate, he’d be more palatable in the general election.

Even at the time, Biden was taking positions farther to the left than ever before; for instance, his health-care plan was far more ambitious than the Affordable Care Act. Yet because it wasn’t Medicare-for-all, it was framed (and derided by some) as the moderate alternative. That reinforced, again and again, the idea that Biden is a cautious moderate.

The second factor in maintaining the idea of Biden’s moderation is that he has very intentionally resisted taking symbolic positions that might sound to some like he’s running to the left. Just as in the primaries he rejected “abolish ICE” when it was suggested, more recently he refused to join calls to “defund the police.”

In both cases, there’s a substantive argument to which Biden might be sympathetic, and one not as radical as the slogan suggests. For instance, most of those who say “defund the police” don’t want to actually abolish the police but, instead, want to redirect some funding so professionals such as mental health workers can handle tasks now performed by police.

But by visibly rejecting the slogan, Biden shows that he’s more moderate than some in his party, and certainly more moderate than leftists who don’t consider themselves Democrats. That provides the former vice president an insulation that has given him the ability to keep steadily moving on policy.

There’s a kind of shift we expect from presidential candidates: In the primaries they appeal to their party with pledges of ideological fealty, then when the nomination is secured, during the general election, they head back to the center. Biden, however, is doing the opposite, in substance if not in rhetoric.

That’s not to say that people on the left shouldn’t be wary; they should. And I should stress that, in nearly every case, Biden is simply becoming more ambitious in pursuing goals he already held. He wanted to expand health coverage, attack inequality and address climate change; now, he’s saying he’ll be more aggressive in doing so.

If Biden is feeling the liberty to be more ambitious on policy because of the situation he finds himself in, it means he could pull back if circumstances change. Which is why just about every progressive activist is greeting his evolution by saying, “This is good news; now we have to keep the pressure on him.”

If he wins in November, they’ll have to worry about the appointments he makes, the legislation he proposes and the executive actions he takes — all of which could be arenas for conflict. Biden might have earned their praise, but he hasn’t yet earned their trust. For that, he’ll have to show that, as president, he can follow through and produce results.

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