These trends are not new. The sole reason there is a Republican majority in the House of Representatives is the massive gerrymandering that took place in the last decades. Democrats have consistently won the "popular" vote for the house, but districts are tactically set to favor Republicans almost across the board.
The districts are this way because while Millennials do indeed skew heavily liberal/Democrat/progressive, they tend to NOT get involved in state and local politics. Republican governors and state legislatures used the last gasps of the dying generation to secure powerful gerrymandered districts ensuring the GOP holds onto the house, at least until the next census.
An interesting side effect, however, is that these artificial superdistricts are such that the Republican is practically guaranteed to win it in the general election. Thus, far-right tea party nonsense candidates can appeal to their local base without much fear of throwing the actual election over to the Democrats. The safer these districts are for Republicans, the further right, racist, sexist, and old they'll skew for the foreseeable future.
Until young people get active in local and state politics, then it literally is a game of just waiting for the current old set in those places to die of old age.