Maryland had been just four days away from clinching some $900 million in federal aid in August when a federal judge ruled to temporarily invalidate environmental approvals for the project. But under President Trump’s plan, projects that don’t yet have complete federal funding agreements would be financed “by the localities that use and benefit from these localized projects.”

Supporters of the project are devastated.

“It was on the one-yard line,” said Art Guzzetti, vice president for policy at the American Public Transportation Association, a Washington-based advocacy group. The proposed budget cuts were inconsistent with the administration’s overall focus on infrastructure investment, he said.

“We’d been so encouraged by all the talk of investing in infrastructure,” Mr. Guzzetti said. “But the budget has been discouraging.”

Elaine Chao, the transportation secretary, has sought to quell concerns. Mr. Trump remains committed to his infrastructure initiative, which he intended to announce later this year, she said last week at an event marking the Transportation Department’s 50th anniversary.

“The president has consistently emphasized that one of his top priorities is modernizing our country’s outdated infrastructure,” she said. “The proposal will cover more than transportation infrastructure. It will include energy, water and potentially broadband and veterans hospitals, as well.”