I think Rubio could have consolidated the anti-Trump vote if he'd come second in New Hampshire and forced Kasich out of the race, rather than losing the momentum he had after Iowa. Trump had his strengths but was also historically unpopular and Clinton was extremely beatable anyway.
A lot of things would have stayed the same. There still would have been three Supreme Court confirmations. It's possible that Rubio would have withdrawn the Kavanaugh nomination after the allegations (if he even nominated Kavanaugh), presumably he wouldn't have faced very similar allegations and so empathised with him automatically. I still think Republicans would have failed on Obamacare. They were way short of the votes to fully repeal and replace it and could never have gotten Susan Collins and the Freedom Caucus to agree on a plan, skinny repeal was pretty much just the inidvidual mandate repeal that passed anyway. There would have been a midterm backlash, but maybe with the suburbanites being happier the House majority would have held. Tax cuts would still have passed. Republicans would have been softer on immigration but the party would still be very resistant to 'amnesty'. Maybe there would have been a deal in 2018, though the opportunity only arose because Trump rescinded DACA.
Without the constant scandals and drama, and by looking presidential during the Covid pandemic, Rubio would have been re-elected. Biden likely would not have even run. Fewer people would have died pointlessly in the pandemic, but it's unclear how far Republicans could have gone in their response. In the second term, Republicans would have faced a huge backlash from a combination of the Dobbs opinion, high inflation (though this could have been reduced if Putin decides not to invade Ukraine), and legislation they could have pushed if they regained a trifecta in 2020. On the other hand, they could have been better off if they achieved better results in the last three elections. The more damaging effects of Trump's presidency are the long-term ones like the undermining of democracy and his character being an acceptable role model for people.
As I said in there, the House in 2018 with Obamacare repeal still being attempted would be a tossup, I think it's a narrow majority either way and Republicans improve the most from irl 2018 in suburban districts. As for 2022, it would be a brutal Democratic wave-six year itch and people would have a lot to be mad with Republicans about even without Trump in the scene.
As for the Senate though, Republicans might hold AZ, possible they do better elsewhere but doubtful. They could have a higher buffer anyway from 2016 Senate. Then in 2020, they retain the Senate easily. It could be lost in 2022 when Democrats finally have a good map and a good environment at the same time.
