Congressional negotiators and the White House appear open to striking a roughly $1 trillion deal on infrastructure. But they are struggling with the hard part — how to pay for it.

As President Joe Biden jumps back into the talks this week, the question of where the money will come from looms large. And time is running short to solve it.

Biden wants to increase taxes for corporations and those households making more than $400,000 a year. Republicans have ruled that out, putting forward alternatives that Democrats find unacceptable. Both sides have said the infrastructure spending should be paid for and not add to the national debt.

It’s a long-standing challenge with no easy solution, one that puts the bipartisan agreement around infrastructure in tension with the nettlesome realities of governing. It’s a problem that has thwarted previous attempts at an infrastructure bill, including during the Trump administration, and their ability to solve it now is likely to determine whether a bipartisan accord is possible.

Senate Republican leader Mitch McConnell has said user fees are the way to go. But the White House and key Democratic lawmakers oppose increasing the user fee that has traditionally funded road and bridge construction, the federal gas tax, even if the increase is just allowing it to rise at the rate of inflation from its current level of 18.4 cents per gallon. The federal gas tax has not increased since 1993.