WASHINGTON — Senator Mary L. Landrieu of Louisiana continued to work on Tuesday to round up votes for the Keystone XL oil pipeline, part of a last-minute effort to help her survive a close runoff and one of the toughest battles of her political career.

Even if the Senate supports building the pipeline in a vote on Tuesday night, President Obama is likely to veto the measure on the grounds that an environmental review of the process remains incomplete.

Nonetheless, the events of this week suggest that after the expected veto, Mr. Obama may eventually approve the pipeline, which would run from the oil sands of Alberta to the Gulf Coast. The project is anathema to the environmentalists who are part of the president’s political base.

White House advisers have repeatedly said that they do not intend to issue a final decision until a Nebraska court issues a verdict on the route of the pipeline through that state. But that decision is expected to come as soon as January, the same month that an incoming Republican-majority Congress can be expected to send another Keystone bill to the president’s desk — one that could be within a few votes of a veto-proof majority.