WASHINGTON (Reuters) - The U.S. Senate Commerce Committee on Monday delayed a vote on the Trump administration’s nominee to be the top auto safety regulator in the face of opposition among Democrats over planned changes to vehicle emissions rules.

Heidi King, who has been deputy chief of the National Highway Traffic Safety Administration (NHTSA) since September, was set to be voted on on Tuesday, but her nomination was pulled from the calendar late Monday because of concerns Republicans would not have enough votes to win approval. That means she will not get a vote until next month at the earliest.

Democrats have raised questions about NHTSA and the Environmental Protection Agency’s backing of a plan to freeze vehicle emissions requirements at 2020 levels through 2026. The proposal could formally go to the White House for review later this week and be made public in early June.

The administration’s draft plan asserts that a 1975 law bars California from imposing its own state emissions rules, as it has long done nonetheless under a series of Clean Air Act waivers.

Automakers including General Motors Co and Toyota Motor Corp want the Trump administration and California to reach agreement to extend national standards rather than engage in a lengthy legal fight. They urged President Donald Trump to try to help reach a deal in a meeting earlier this month.

California Air Resources Board chief Mary Nichols is set to be in Washington later this week for a meeting with Trump administration officials.

Earlier this month, California and a group of 16 other states challenged the Trump administration’s decision to revise the vehicle standards. The states challenged EPA’s decision in April to declare U.S. vehicle emissions and fuel efficiency rules through 2025 “not appropriate.”

The rules adopted in 2012 under Democratic President Barack Obama sought to double average fleet-wide vehicle fuel efficiency to about 50 miles (80 km) per gallon by 2025, but included an evaluation due by April 2018 to determine if the rules were appropriate.

King also faced questions from senators at her confirmation hearing on the state of the record-setting Takata air bag inflator recall and the agency’s oversight of self-driving vehicles.