As the midterm campaigns heat up, Democratic candidates are turning to the White House for help and an injection of star power. But it is the first lady, not President Barack Obama, who in some places is emerging as the more sought-after campaigner.

Some Democrats contend that Mr. Obama's sagging popularity limits where he would be helpful this election cycle. Bringing higher approval ratings and a less-partisan pitch, Michelle Obama already has hit the trail, appearing earlier this month in Georgia with Senate candidate Michelle Nunn. Next month, the first lady's campaign travel will include a visit to Iowa, where Democratic Rep. Bruce Braley is battling Republican Joni Ernst for an open Senate seat.

Mr. Obama has appeared at dozens of fundraisers for Democrats this year, but he won't begin campaigning for congressional and gubernatorial candidates until next month. The White House has yet to announce any campaign events for the president, but a senior administration official said Mr. Obama would focus on select races in blue and purple states.

"He's going to do everything he can to help the candidates," the official said. "In some cases, that may mean staying out of the way."

In a few states, Democrats say Mrs. Obama would be the more effective campaigner.