The decision by the Federal Energy Regulatory Commission (FERC) was handed down in a unanimous verdict by its five members, a majority of whom belong to the president's Republican Party.
Energy Secretary Rick Perry had in September proposed providing federal aid to nuclear and coal power plants with at least 90 days' worth of production capacity, arguing the move was necessary to make the national grid more resilient in case of extreme events.
Both sectors have seen their share of the energy market diminish in recent years, losing out to oil, natural gas and renewables -- which had all opposed Perry's plan.
There are currently only two nuclear reactors under construction in the US, in addition to the 99 in service. Coal is also facing a crisis, and Trump made reversing its decline a major campaign pledge.
In announcing its decision, FERC cited an existing department study's findings that "changes in the generation mix, including the retirement of coal and nuclear generators, have not diminished the grid's reliability or otherwise posed a significant and immediate threat to the resilience of the electric grid."
But it sought suppliers to provide within 60 days reports related to resilience concerns and issues the commission had identified.