Giono//
The 2020 Senate map makes it virtually impossible for a Democratic president to get anything other than a Republican senate. It's 53-47 right now and the Republican incumbents from potential swing states are all running in a general election year. With the strong incumbent-bias in senatorial elections, in particular during presidential years, anything other than Republican wins in North Carolina, Georgia, Iowa and Arizona would be massive shocks. Mid-terms always have low turnouts for the governing party.
Also, Alabama is up for grabs and given there's zero chance Roy Moore gets nominated again, the Republicans will most likely take that one back with ease. I'd say 54-46, if not more will be the majority after 2020 unless something incredible happens before then. If Trump has a strong election year against a weak Democrat, then maybe one of Minnesota, Michigan or New Hampshire could potentially be picked off too. Maine then? Well, Trump is strong in Maine relative to other Republicans and Susan Collins is rather bipartisan and a long-term incumbent. I don't think she's going anywhere given she's always gotten crossover support.
So, say there's a 54-46 outcome, that definitely puts a lot of pressure on the Democrats in 2022 during those mid-terms, having to win back all of Wisconsin, Pennsylvania, North Carolina and Florida just to make it a tie, if Trump is re-elected. If there's a Democrat in office, then they can forget about it. So, no matter who the Democrat is, the Republican roadblock will always be there for now.
Why is that then?
Well, due to the makeup of the senate of two votes per state, the rural nature of the modern-day Republicans are starting to show off more and more. They've yet to get to 2-0 in Montana and West Virginia, but for Indiana, Missouri and North Dakota, the lost Democrat seats are extremely unlikely to be won back due to the demographics. It's the same in Iowa really. The Democrats need to have at least one in Texas and Florida to have a chance to control the senate in the short term and that's hard to achieve. The way states trended in 2016, it's not hard to conceive Minnesota and New Hampshire being "tied" states in the near future as well. Ohio would've been had the election been held in either 2016 or 2020, Brown's win was rather weak even in an opposition party midterm year.
Any Democrat president's hands will therefore be tied and supreme court picks really hard to get through for them, if not impossible, it's quite simple really.