There was a school of thought that said former Vice President Joe Biden would begin to sink in the polls the moment he announced his candidacy for the Democratic presidential nomination. Biden's first day in the race, the thinking went, would be his best day.

In fact, the opposite has happened. Since formally becoming a candidate on April 25, Biden has shot up in the polls. On announcement day, Biden held a 6.3-point lead over second-place Sen. Bernie Sanders in the RealClearPolitics average of polls. Today, that lead is 23.5 points. That is a big change.

Polls do not tell us who will win an election months from now. But they do tell us what is happening at this moment. And at this moment, Democratic voters, who are sometimes said to be moving left and itching to transform the United States with a "Green New Deal," "Medicare for All," and through-the-roof taxes on the rich, are in fact responding to a decidedly more centrist appeal.

That appeal, from Biden, is a promise not to fundamentally remake American society but to restore things to the way they used to be. And "the way they used to be" means before President Trump.

Obviously, Democratic voters want to replace a Republican president with a Democratic president. But they are especially dismayed by Trump — and some, driven by increasingly strident news coverage, seem to have gone nearly 'round the bend about him.

But for some center-left Democrats, the solution to the Trump Problem — that is, the fact that Trump is president — might not be the "Green New Deal" or "Medicare for All." It is to restore the pre-2017 order in American politics. And Biden, Barack Obama's vice president from 2009 to 2017, is the physical embodiment of that old order.

That is what Biden promises. Nearly every day, he repeats some version of his core campaign pledge: "I want to restore the soul of this country."

Biden's unexpected choice of Charlottesville, Va., as the theme of his announcement was a way of saying that something has gone terribly wrong in the U.S. and that he wants to return to the pre-Trump past. Addressing a real or imagined moral crisis is one way for an opposition candidate to run against an incumbent president whose term has brought solid economic growth, low unemployment, and higher wages.

How long will Biden's lead last? Who knows? There is simply no telling how the Democratic race will play out. In the last two Republican nomination contests, we saw one race, in 2012, in which several candidates alternated holding the lead before Mitt Romney finally won. In the other, in 2016, we saw Trump lead a big field virtually the entire time. Now, with an even bigger Democratic field, the race dynamics are not yet clear.

Plus, for Biden specifically, there will always be the issue of age. Biden will be 78 years old on inauguration day 2021. That is the same age Trump would be upon leaving office, should he serve eight years. But Biden would be just beginning his presidency nearing the age of 80. That is totally uncharted territory in U.S. history. (By the way, one other candidate, Sanders, is even older.)

Even if Democrats want to restore the old order, they might decide a younger candidate should do the job.

They might also want a candidate without Biden's record of fizzling out in presidential campaigns. In his first run for president, in 1988, Biden withdrew amid a plagiarism scandal before any votes were cast. In his second run, in 2008, he quit after finishing fifth in the Iowa caucuses. So, he has run twice and never even made it to the New Hampshire primary.

Now, though, Biden stands ahead of the field. Democrats know how old he is, they know he has lost in the past, and they still like him.

There's a truism that elections are always about the future, not the past. That's often the case. But what if it isn't this time? A lot of political truisms did not hold up in the 2016 election, which was won by a man with another promise of restoration, to "Make America Great Again."

Now, many Democrats seem happy to support a candidate who pledges to take them back a few years. Again, that could change, but for the moment, it shows how many Democrats yearn to return to a time before Trump.