At the beginning of the year, the debate concerning the Senate was mostly whether the Republicans would gain or lose any seats compared to their number of 53. Now, it is a debate of whether they can keep their majority or not. 538 (one of the best statistical prediction models out there) gives the Democrats a 2/3 chance of reclaiming the Senate.
What happened? Well, Covid-19 has hurt not only President Trump’s chances but also the Senate, since the Republicans are tied to him. And the map for this year is extremely favorable to Democrats. Of the contestable races, there is only one they will likely lose (Alabama) and one that is a possible loss (Michigan). But they could be more than compensated by the races of Colorado, Arizona, Maine and North Carolina (and there are still other possibilities like Iowa, although not as likely).
And here is the scary part for Republicans- even if they somehow defy the odds and maintain their majority, 2022 will be even worse. Their incumbents in NC and PA are already planning to retire. This only makes the map worse, since they are defending other seats in Wisconsin, Ohio and Florida. If Donald Trump does win, 2022 can be expected to be a disastrous year for Congressional Republicans (this phenomenon is called the Six Year Itch, when a President’s party is voted against in their sixth year of office).
Now, if Joe Biden were to win, then perhaps things could be different for Republicans. Just as states they expected to win are now swing elections, so could the tables turn on Democrats if Joe Biden won. It’s a funny thing about American politics. They will vote for the President’s party, and then two years later decide they are sick of him/her and vote for the opposing party in Congress.