One thing is that the Democrats will not win the Senate next year. Republicans would have to lose 3 seats and they only have 2 vulnerable (Arizona and Nevada). There is no way they are losing any of their other seats, plus they only have 8 to defend as opposed to 25 for Democrats.
It will be difficult, but if Trump stumps for a couple or refuses to stump against vulnerable dems, that may be enough. Even if all he does is that and the Dems lose, that's not on him and any deal he made with Schumer and Pelosi will still be good.
Remember McCain, Graham and Murkowski could all flip.