It’s not supposition at all. It’s math and history. In the House the Dems only have 188 seats, given the average re-election rate is around 95% we’re talking about a 10 seat gain being pretty much the best the GOP can hope for. In the Senate the Dems only HAVE 10 seats on the line, again retention rate is over 90% so we’re talking maybe 2 seats, maybe 3. Gaining seats is harder the more you already have, it’s math, with a finite number of seats the fewer seats your opponent has the fewer seats you CAN gain.
And history shows setting the party policy really is just paper.
We’re not talking about polling, we’re talking about coattails.