Because, as ABC shows, that's not true right now.
If a majority of Millennials right now want a GOP Congress, then Democrats, starting with the President and moving down, are doing at least one something wrong politically. (By politically, I'm talking about the art of winning at politics, not the skill of governance, though they may be doing stuff wrong there, too.)
Millennials who said they will “definitely be voting” favor a Republican-led Congress 51-47 percent, according to the poll. An ABC News/Washington Post poll released earlier this week found that roughly half of Americans plan to vote for Republicans this election, 50-44 percent.