A popular federal air quality program that targets pollution from older diesel engines would get minimal EPA funding under the administration’s 2020 budget proposals.

The Trump administration’s proposed $10 million for diesel emissions reduction in the fiscal 2020 Environmental Protection Agency budget request comes as Republicans and Democrats focus this year on reauthorizing the program.

The grants have garnered consistent support from engine manufacturers such as Caterpillar Inc., as well as environmental groups.

The proposal’s $10 million represents a roughly $63 million cut from 2019 and 2018 funding levels. The fiscal 2019 levels reflect the appropriations level for EPA...