A review of past midterm election-year data by Gallup "suggests that even early predictions can give insight into how next year's elections might go. The size of the Democratic lead among registered voters in the months ahead will be a key indicator of whether the election will be competitive. If Republicans are close to being tied or are ahead, it will suggest the possibility of a strong Republican year unless Democrats have an unusually strong and disproportionate turnout advantage."
With their most recent generic ballot showing Democrats leading Republicans by just two points, 46% to 44%, the "possibility of a strong Republican year would be consistent with history."