The White House is set to step up pressure on some key Republicans in hopes of winning support for comprehensive immigration reform. But it’s shaping up to be a struggle, based on interviews with the senators President Barack Obama has approached on the issue. The administration is starting with a pool of 11 Republicans who voted for immigration reform in 2006. Subtract a few who are dead-ends — such as John McCain, who faces a tough primary in Arizona — and that leaves the White House zeroing in on several others, including Dick Lugar, Judd Gregg and Lisa Murkowski and...