WASHINGTON (AP) — Eleven Senate Democrats are urging President Barack Obama to approve the Keystone XL oil pipeline by the end of May.

The senators say the five-year review of the Canada-to-Texas pipeline that would pass through South Dakota has been “exhaustive”. The letter sent Thursday says approval is needed to ensure the pipeline doesn’t miss another construction season.

But politics is likely a larger factor. Six of the Democrats who signed the letter face challenges this year. They are Mark Pryor of Arkansas, Mary Landrieu of Louisiana, Mark Begich of Alaska, John Walsh of Montana, Kay Hagan of North Carolina and Mark Warner of Virginia.

Democratic efforts to keep control of the Senate could hinge on those races.