The tenuous 45 percent to 40 percent preference for a Democratic Congress reverses the finding a month ago on the same question: 44 percent for Republicans and 41 percent for Democrats. The new readout came as the economy continued showing signs of improvement and the tumultuous battle over the health care law that President Barack Obama finally signed in March faded into the background.

“To the extent that Democrats can focus on job creation rather than health care, they tend to do better,” said Jack Pitney, a political scientist at California’s Claremont McKenna College.