The most important paragraph on who wins Congress in every election in the forseaable future comes from Jay Cost:
If “safe Democratic” states yield 204 electoral votes, they are only good for 32 senators. Meanwhile, “safe Republican” states are only good for 182 electoral votes, but provide 44 senators. That leaves 24 senators from 12 states that either side may win in presidential years. If both parties won all their safe seats, and they split the seats in contested states, we would see a GOP majority in the Senate of 56-44.The advantage the GOP enjoys in the Senate is not as decisive as its edge in the House, as Democratic senators in red states have done a better job of holding on than their House counterparts.
That's a paragraph on the Senate plus a sentence to cover of the House of Congress. It speaks volumes. As Cost notes,
What accounts for the GOP’s success in the House and its potential in the Senate? The answers parallel the explanations for Democratic strength in the race for the presidency: It gets down to structure....The problem for the Democrats is a combination of law and geography. The 1982 amendments to the Voting Rights Act require the creation of majority-minority districts whenever they can be drawn with reasonable lines. In effect, state legislatures are required to concentrate Democrats in a handful of districts, while dispersing GOP voters across the remainder.
That's a sign of continued electoral bliss for Republicans in the House and Senate. But it also represents a true challenge at the presidential level as the GOP is playing from behind. They'll need to bring other states the Democrats have into play more often and more effectively or they'll be perennial losers. Which means eventually the SCOTUS slips to Democratic control. And that in turn allows Democrats to erode the GOP strengths in the legislative branch of government.