Extreme partisanship, which took hold long before Trump took control of the G.O.P., is part of the answer. It was clear through most of the Obama years that Republicans wanted to prevent good things from happening on a Democratic president’s watch. Under Obama, G.O.P. legislators squeezed federal spending after they took control of the House, supposedly because they were worried about government debt, only to open up the taps once Trump took office.

There was also an element of reflexive opposition to government spending in general, no matter how obvious the need. Mitch McConnell, as Senate majority leader, didn’t do much to oppose Trump — but he did put the brakes on infrastructure spending.