President Barack Obama is intervening in a Senate fight over the Keystone XL oil pipeline and personally lobbying Democrats to reject an amendment calling for its construction, according to several sources familiar with the talks. The White House lobbying effort, including phone calls from the president to Democrats, signals that the vote could be close when it heads to the floor Thursday. The president is trying to defeat an amendment that would give election-year fodder to his Republican critics who have accused him of blocking a job-creating energy project at a time of high gas prices.