“Here is what I know to be true, Mr President,” Vice-President Kamala Harris said on Monday, as she addressed Joe Biden. “You are equal parts believer and builder. And because you are, we are all better off.”

Biden is a believer in the ability of the American people, US democracy and Democrats and Republicans to work together and get things done. It was the theory of his candidacy for president.

But his faith has been sorely tested since he took office.

Reality keeps mugging Biden and yet he comes back for more, his convictions apparently unshaken. He refuses to be disappointed in America. His reward came on Monday when he signed into law a bipartisan trillion-dollar bill to repair roads, bridges and other infrastructure.

The president told a gathering of about 800 mayors, governors and workers on the White House South Lawn: “The bill I’m about to sign is proof that despite the cynics, Democrats and Republicans can come together and deliver results. We can do this. We can deliver real results for real people.”

Only the harshest of cynics would deny him this victory lap for the biggest public works bill since former president Dwight Eisenhower created the interstate highway system in 1956. “Joe! Joe! Joe!” chanted supporters as the sun set, casting its light on him but leaving most of the crowd in the shade as chill winds blew.

But it did not take a cynic to notice the clues that Washington still has a long way to go to heal its deep, dysfunctional divisions and live up to Biden’s idealism. This might be a new dawn of bipartisanship. Or it might be its last gasp.

Infrastructure, after all, is a low bar to clear. Foreign visitors to the US are stunned to find the most powerful nation the world has ever known plagued by potholed roads, crumbling bridges and clapped-out airports. China and others are racing ahead. America had to act because it couldn’t not act.

Six years ago, when Congress last approved a significant renewal of federal highway and other transport programs, it was backed by nearly every Democrat and robust majorities of Republicans.

This time, the $1.2n infrastructure bill cleared the Senate 69-13 with Republican support, but scraped through the House last week with just 13 Republican votes. And they have paid a political price.

Donald Trump, the former president, lambasted them. Marjorie Taylor Greene, an extremist member of Congress, branded them “traitors” while tweeting their names and office telephone numbers.

Congressman Fred Upton of Michigan said his offices received dozens of threatening calls following his yes vote. That included one obscenity-laced rant in which the caller repeatedly called Upton a “traitor” and expressed hope that he, his family and aides would die.

If the Maga (“Make America Great Again”) wing of the Republican party was hoping to intimidate party members so they would stay away from Biden’s shindig, they appear to have succeeded. Only a sprinkling of congressional Republicans were present to hear this paean to bipartisanship.

Mitch McConnell, the Senate minority leader who voted for the bill, was notably not among them, apparently unwilling to enhance Biden’s political victory (the president thanked him in his absence, all the same).

Instead there was the Republican senator Rob Portman of Ohio, who is not running for re-election and so has nothing to fear from the vengeance of Maga world.

Biden joked: “Senator Rob Portman is really a hell of a good guy. I’m not hurting you, Rob, because I know you’re not running again.”

This was not a Trumpian celebration of fireworks, military jets roaring overhead and tanks parked on the lawn. Nevertheless, the Rose Garden colonnades were bedecked with the Stars and Stripes and the flags of all 50 states were lined up at the south portico.

Portman, Senator Kyrsten Sinema (neither wearing a coat despite the cold), the Senate majority leader, Chuck Schumer, and House speaker, Nancy Pelosi, strode across the South Lawn as a fanfare played from a brass band.

Sinema, a centrist Arizona Democrat, told the gathering: “How many times have we heard that bipartisanship isn’t possible any more or that important policy can only happen on a party line? Our legislation proves the opposite and the senators who negotiated this legislation show how to get things done.”

But her prominent role summed up the ambiguity of the moment. Sinema has enraged many on the left by her enigmatic and intransigent approach to part two of Biden’s agenda, known as Build Back Better, which proposes $1.75tn in social and environmental spending. Republicans are uniformly opposed.

That deadlock is not helping Biden’s approval rating, hovering around the 40% mark despite jobs growth, nor doing much to dispel the fear that the rot has set in for good in the body politic.

But for one afternoon at least, Biden could be Biden, giving thanks to absent friends and reasserting his belief in America.

“I ran for president because the only way to move our country forward is through compromise and consensus,” he said.