That staple of US public life, the Joe Biden gaffe, has been elusive since his inauguration as president in January. With luck, its return will not scupper the infrastructure bill he agreed with senators last week. Biden has withdrawn remarks that seemed to peg the bill to a more contentious one on social spending and tax increases. If Republicans and conservative Democrats accept his assurances, $1tn of investment could be enacted soon. Not just bridges and power grids will benefit. A White House that was struggling to match the pace of its first hundred days would have new life.

There will also be some regrets. Next to initial hopes, the bill stands emaciated. Its headline value is less than half the figure that Democrats first touted in March. Even that overstates the amount of new spending, which, once leftover stimulus cash is excluded, is nearer $600bn. Spread across a continental land mass over eight years, the sums are hardly nation-changing. Do not expect the US to gleam with Chinese-style trains, Germanic roads and Dubai-grade ports in 2030.

Another concession is over the funding. The bill provides for a better-resourced Internal Revenue Service, which might extract more in receipts from existing taxes. But Biden had wanted marginal rates themselves to go up, at least for corporations and the highest earners. He will now have to plot other routes to that end. His Congressional colleagues and the wider left are already devising ones.

Even in its reduced state, the bill is more than Donald Trump had to show for years of talking up infrastructure. In the end, however, Biden’s achievement is less the content of the deal than its bipartisan manner. It honours the implicit promise of his entire presidency: that only a moderate in tone and thought, half a century steeped in Washington, can make the system work in an age of hot-heads and insurgents.

It also serves his grander aim: to show the world’s autocracies, and nations tempted by their model, that democracies can still “do significant things”. Framing infrastructure spending as part of the race against China helped in persuading Republicans.

The trick is to do it again. After his pandemic relief bill passed without Republican votes, Biden needed a bipartisan moment. The mystery is whether an accord on something as universally popular as infrastructure can extend to voting reform, climate change and other emotive fronts.

A tiny number of people will decide. It has taken a while for the implications of the last Congressional elections to become entirely clear. In a Senate that is almost evenly split, any lawmaker who is open to voting with the other party has disproportionate clout. Among those so empowered are the conservative Democrats Kyrsten Sinema and Joe Manchin. The moderate Susan Collins has a similarly decisive role in the Republican caucus. In few western capitals were politicians loosely describable as centrists so weak so recently. They are now, if not ascendant, then at least waking up to the power conferred on them by Congressional arithmetic.

Of course, the centrist position on a given subject is not axiomatically the proper one. The left is correct that, if the US is to have a larger state, it cannot keep ducking — as the likes of Manchin seem to — the question of tax. Still, the elevation of a few non-partisans in national life is precious for reasons that go beyond policy. At least until the midterm elections of 2022, it promises the tonal softening of a shrill and vicious Washington. A civilised politics is the ultimate infrastructure.