Many of the 85,000 dams in the United States are so old — an average of half a century — that every time one is repaired, two more become dangerously weak. Cities across the country discharge billions of gallons of untreated wastewater into rivers and lakes, and more than a quarter of all bridges are either deficient or obsolete.

The statistics are both frightening and familiar, though they tend to come up only in the “crumbling infrastructure” articles that appear after major disasters. In practice, government — with its lack of cash and consensus — keeps most of these projects on distant back burners until people actually lose their lives.

And then a disaster occurs — like the one in Japan, which was a reminder that even a well-prepared small country can suffer terribly from a natural disaster. The hazards are even greater for a sprawling one with a long history of indolent maintenance and planning.

Last week, though, a bipartisan group of senators came up with a promising idea to get some of these projects started, and very possibly put thousands of people back to work by doing so. The proposal, to create an infrastructure bank that would lend out seed money, represents a refreshing break from the extremist culture of cutting for the sake of cutting that grips Washington and so many state capitals. That culture blocks vital investment just to avoid sensible tax increases.

The proposal was presented by John Kerry, Democrat of Massachusetts; Kay Bailey Hutchison, Republican of Texas; and Mark Warner, Democrat of Virginia. The bank would lend money to build big-ticket transportation, water and energy projects that have a clear public benefit. The loans, or loan guarantees, would be designed to attract private capital as well. In fact, at least half a project’s financing would have to come from the private sector. As much as $640 billion could be leveraged this way over the next decade, proponents say.

The bank would initially be funded with $10 billion from the treasury, which would be given out as loans, not grants. To make that possible, the bank would invest largely in projects that generate money, like toll bridges and tunnels, water systems backed by ratepayers, and energy projects built by utilities, governments or corporations. An independent, bipartisan board appointed by the president and Congress would choose the investments and oversee construction, audited by an inspector general and the Government Accountability Office.

By providing low-cost capital to states, cities and authorities, the bank would help these strapped governments kick-start projects that are now unaffordable, while attracting investments from pension and private-equity funds that are looking for stable money-generating ventures in which to invest. “We can either build, and compete, and create jobs for our people,” said Mr. Kerry, “or we can fold up, and let everybody else win. I don’t think that’s America.” The bank was backed by unions and the U.S. Chamber of Commerce.

The idea builds on one that President Obama has proposed, a $30 billion bank limited to transportation projects that would also make grants. It is designed to be more palatable to lawmakers who are politically averse to spending, but already conservatives are railing against what some have called a “boondoggle,” a phrase used to demonize virtually any public investment.

What will these opponents tell voters when the dams break and the bridges fall? Before more lives are lost, lawmakers should ask themselves whether they used their public office only to slash spending (and taxes for the wealthy), or to spend money wisely.