Some actions by political leaders are capricious. Some can be shortsighted. And some are sheer lunkheaded. President Trump hit the trifecta last week when he encouraged the House speaker, Paul Ryan, to scrap start-up money for an additional rail tunnel between New York and New Jersey, a project essential to the economic health not only of those two states but of the entire country.

Worse yet, the president’s action bore no relation to objective analysis of the region’s infrastructure needs. Accounts in The Times and The Washington Post said he did it to spite the Senate Democratic leader, Chuck Schumer of New York, whose sin is failure to fall in lock step with Mr. Trump on a variety of issues.

This is foolishness. Republicans and Democrats alike broadly agree on the essentiality of the so-called Gateway tunnel, described by many officials as the most urgently needed infrastructure project anywhere in the United States. It will not come cheap, with $11 billion required for the first phase and an estimated $19 billion more needed to finish the job. “People get frightened by the cost,” said John Banks, president of the Real Estate Board of New York. “But the alternative is worse.”

Existing tunnels under the Hudson River are more than a century old and stressed by damage from Hurricane Sandy in 2012. Losing one of those tubes would greatly reduce train capacity, to devastating effect. With the metro region said to account for about 10 percent of the national economy, it doesn’t take a seer to appreciate that such a blow would be, to borrow from Mr. Trump when he’s in high dudgeon, a disaster.