Just how transformational is the Biden presidency?

Enthusiasm for President Biden’s ambition is rampant among progressives. In the first 100 days of his presidency, he has inspired premonitions of the second coming of Franklin Roosevelt. In his address to Congress this week, Mr. Biden himself invoked the parallel, “turning peril into possibility.”

And no wonder: After breaking through in the Democratic primaries as a centrist, Mr. Biden has surpassed his party’s expectations for the scale of his vision and moved sharply to the left in his early days in office.

It is not easy to explain Mr. Biden’s “radicalism.” For the most rapturous, a big-spending champion of a new welfare state has arisen from a cautious market-friendly centrism. The American Rescue, Jobs and Family Plans augur a “decisive break” with the era of small government, and the pandemic’s exposure of American inequalities and the evolution of our experts anticipate a new era in our politics.

That is not the whole story. The Democrats’ newfound tolerance of deficits for the sake of relief, infrastructure and care does move beyond the austerity economics of the last several decades. But as the journalist David Dayen points out, it has not affected a budget proposal promising to “restore” discretionary nondefense spending to levels that are still less than those during the Ronald Reagan era (as a percentage of gross domestic product). Mr. Biden’s rebalancing of tax fairness for individuals takes the country back, as the president acknowledged Wednesday, to George W. Bush levels of under 40 percent for the top tax bracket, not Roosevelt levels of 94 percent at their height or even pre-Reagan levels of 70 percent.

If Mr. Biden’s first 100 days differ so much from the New Deal, however, the fear that motivated Democrats back then is the best explanation of their early actions and talk now, especially when it comes to rethinking the American social contract. In his first inaugural, Roosevelt warned against the fear of fear itself. The truth, as the New Deal historian Ira Katznelson memorably emphasized, is that anxiety drove many of the innovations of the era, from the contraction of class inequality (and high tax rates) at home to the militarized stance toward enemies like the Nazis abroad. But terror of risks to stability and wealth, as much as freedom, lay behind a redefinition of social fairness and the rise of a new kind of state.

That fear can drive reform, while also limiting and marring it, is what we need to consider once again. What Democrats are afraid of best explains what they are doing, and where they will stop — and that may be the problem.

Mr. Biden’s foreign policy staffers have been most cleareyed about their challenges. Secretary of State Antony Blinken acknowledged that “Americans have been asking tough but fair questions about what we’re doing, how we’re leading — indeed, whether we should be leading at all.”

Brian Deese, Council of Economic Policy head, is also entirely open that losing voters again in a closely divided country keeps him up at night: “Your ability to sustain good policy is connected to your ability to sustain political support for that good policy.”

With a higher minimum wage on hold (Mr. Biden did order a $15 minimum wage for federal contractors) and the fate of Mr. Biden’s proposed increase in the corporate tax rate unknown, much of the history the Democrats are writing is now up to Congress. The red meat of Mr. Biden’s proposals will look very different after the sausage grinder of the legislative process.

But both the generosity and limits of the reformism of fear always depend on what exactly reformers find terrifying — and what they think will lead to safety. The threat of electoral loss will wane as soon as it seems less credible that Donald Trump or someone like him can capitalize on elite failures. Even as long as that fear lasts, it can as easily lead to optical or rhetorical change as drive structural reform. And fear conditions the kind of government investments chosen from the policy menu.

It is not just politicians angling to stay in power whose fear we need to realistically assess. Much depends, too, on the fear levels of the donors to whom politicians answer. In the 20th century, the carnage of war and masses enraged by depression — and pushing for labor rights through street action and union politics — once led the rich to redistribute to the rest more willingly. But it is unclear how genuinely and how much our generation’s wealthy, whose big donations made a big difference for Mr. Biden in the 2020 election (much as well-off suburbanites did with their votes), are terrified, or how far the president will ultimately shape policy to their demands.

If the New Deal shows that fear can motivate reform, finally, it also reminds us that it can lead that reform awry. The one big change in foreign policy Democrats are making to their pre-Trump understanding of what a “rules-based international order” requires concerns China, especially in trade policy. That Democrats are embracing so floridly the model of great power competition with China that Mr. Trump embraced — even perhaps a new Cold War — suggests they know they need more than the anxiety they will lose again or the threats to democracy associated with the right (and confirmed by the Jan. 6 assault on the Capitol) to keep themselves and many in their audience motivated.

“When we think about infrastructure,” said Mr. Deese, “a lot of it is in contraposition to what [Biden] is seeing China doing.” As Mr. Biden himself remarked on Wednesday: “China and other countries are closing in fast.”

The New Deal truly changed America when it ended not in a welfare but in a warfare state — and that proved a catastrophe for kind of ambitious reform Mr. Biden says he wants. Apprehensive competition can bring distortion, excess and manipulation, and not merely goad policymakers to change for the better or constituencies to support that change. Ambition can spring from rivalry, but competition, as the first Cold War with the Soviet Union showed, can also limit reform and lead to collateral damage and disastrous mistakes.

The limits of Mr. Biden’s ambitions are the limits of the reformism of fear. For all the good it can provoke, a politics driven by threats from angry voters, domestic uprisings and foreign states can never break the American impasse. Only hope and higher ideals can.