WASHINGTON — Barack Obama’s campaign has approached Illinois Rep. Rahm Emanuel about possibly serving as White House chief of staff, officials said Thursday as the marathon presidential race entered its final, frenzied stretch with a Democratic tilt. The disclosure came as Republican John McCain, in need of a comeback, focused on pocketbook issues amid fresh signs of a recession. ‘‘Ohio is hurting now, people in Ohio are having trouble staying in their homes, keeping their jobs,’’ he said as he set out on a two-day bus tour of the state.