WASHINGTON (AP) -- The Senate has voted to move forward with a bill setting aside more than 2 million acres in nine states as wilderness.

The 66-12 vote during a rare Sunday session marks an early partisan showdown that has threatened to derail pledges by Senate leaders to work cooperatively as a new administration takes office.

The bill is the largest expansion of wilderness protection in 25 years. It has bipartisan support and would include California's Sierra Nevada mountain range, Oregon's Mount Hood, Rocky Mountain National Park in Colorado and parts of the Jefferson National Forest in Virginia.

The Senate vote limits Republican stalling tactics and enables the bill to move forward. Senate approval is expected later this week.