Even if Casey wins, the Republicans still have the majority. But it will help in the future. Via NBC News:

Pennsylvania’s Senate race is heading to a recount, which the narrow margin between Republican Dave McCormick and Democratic Sen. Bob Casey triggered automatically under state law.

Secretary of the Commonwealth Al Schmidt announced Wednesday that the unofficial results showing a narrow race have led to a recount. Schmidt noted in his announcement that counties must begin the recount no later than Nov. 20 and that they must complete the process by noon Nov. 26. The results will not be published until Nov. 27.

Just over 29,000 votes, or 0.4% of the total votes cast, separate the two candidates, with McCormick narrowly leading. Any margin under 0.5 percentage points triggers a recount in Pennsylvania. NBC News has not yet projected a winner in the race.