President Joe Biden is racing against time.

“We have to move now,” Biden said last week as he unveiled his giant $2 trillion-plus economic plan. “We can’t delay another minute.”

Why is a 78-year-old president once considered a cautious moderate suddenly going for broke?

Because he knows his time is short.

Like most new presidents, Biden is enjoying a modest honeymoon in the eyes of the public, but he knows from experience it won’t last.

His job approval rating has been steady at about 54%, a higher number than his immediate predecessor ever saw. The public liked his $1.9 trillion COVID-19 relief bill a lot; in some polls, 70% or more approved.

Surveys suggest the public likes the core of his new proposal, too: a plan to spend big on infrastructure paid for by higher taxes on corporations.

But poll numbers often prove evanescent.

Republicans have only begun to campaign against the new plan, which stuffs a long Democratic wish list inside a shamelessly elastic definition of “infrastructure.” (Home care for the elderly is a worthy goal, but should it count as “human infrastructure?”)

The president’s historical memory is prodding him to aim high and move fast. His aides often say Barack Obama made a big mistake by wasting months fruitlessly seeking GOP support for his health care proposal.

The political clock ticks even more loudly for Biden. Most political prognosticators say Nancy Pelosi’s Democrats are likely to lose their majority in the House of Representatives next year, and history is on their side. The president’s party has lost an average of 22 seats in every midterm election since World War II;Democrats could only lose five.

If the GOP regains the House, Biden’s window for passing ambitious legislation will abruptly slam shut, just as it did for Donald Trump at his two-year mark in 2018 and Obama at his in 2010.

During his two years of one-party control, Obama passed three major bills: an economic stimulus package, Obamacare and the Dodd-Frank financial reforms. Trump, whose self-proclaimed genius as a deal-maker deserted him in Washington, passed only one: his 2017 tax cut. Biden is hoping to outdo Obama’s record with much bigger dollars.

Now add a more personal factor: the the 78-year-old Biden’s intimations of mortality.

That helps explain why Biden’s professed willingness to seek bipartisan consensus with Republicans has been swept away, in practice, by his countervailing willingness to take the quicker route of passing bills with only Democratic votes.

In the end, the president’s big infrastructure plan may prove too sprawling and expensive to pass.

The tax increases he wants may prove impossible to push through.

The polls could turn south as Republicans marshal opposition.

His own Democrats, with razor-thin majorities in both the House and Senate, may not hold together.

But the president has at least one thing right: He needs to assume that his time is short. When it comes to ambitious legislation, the Biden era is likely to last only two years, like the Trump era that preceded it and the Obama era before that.

Doyle McManus is a columnist for the Los Angeles Times.