In 2008, then-candidate Barack Obama's political coattails extended across the country. But heading into this year's midterm elections, Democrats face a tricky task of where to deploy their party chief on the campaign trail as they try to hang onto majorities in both houses of Congress.

President Obama's record over the past 17 months has been mixed: His party won a series of special House elections, but he put himself on the line as Democrats lost a pair of high-profile gubernatorial contests in New Jersey and Virginia. Then there was the special senatorial election in Massachusetts, where even a personal last-minute pitch by the president couldn't stop Republican Scott Brown from winning the seat long held by liberal lion Sen. Edward M. Kennedy.

Still, even as poll numbers have dropped from their lofty post-inauguration levels, campaigns have requested varying levels of involvement from Mr. Obama — from large rallies and local fundraisers to smaller, more targeted efforts such as "robocalls" in a certain part of a district. And there are places like California, where Mr. Obama is both a big draw and a stellar fundraiser, racking up as much as $1.5 million in his last visit on behalf of incumbent Democratic Sen. Barbara Boxer...