Forecasting models for Senate outcomes are scarce but handicappers like the Cook Political Report, which tends to be fairly conservative in its assessments, are now making Democrats favorites to take the Senate. Obviously, that's a huge deal. Check out this excellent deep dive on the Senate situation from Jessica Taylor from the Cook Political Report: Note they now consider Iowa, Montana and Georgia (Perdue) as tossups.
"With just over 100 days until Election Day, the political climate appears dire for Republicans across the board. President Trump is the decided underdog against former Vice President Joe Biden in our Electoral College ratings and Democrats could end up expanding their House majority.
That leaves the Senate as Republicans' firewall—the final barrier to unified control for Democrats in 2021. While GOP incumbents are trying to run races independent of the president, if Trump’s polling numbers remain this dismal come November, that’s an unenviable and likely unsuccessful strategy, according to several top party strategists. As of now, Democrats are a slight favorite to win the Senate majority.
“Something remarkable would have to happen for Republicans to still have control of the Senate after November,” remarked one GOP pollster. “It’s grim. There’s just so many places where Democrats either have the upper hand or are competitive in states that six months ago we wouldn’t have considered at risk.”...
Taken together, that’s not just a perfect storm for Democrats, but perhaps a perfect tsunami. “The bottom fell out for us at the end of May and June,” with worsening numbers continuing into July now, one national GOP strategist looking at polls across the map bemoaned.
Ultimately, every day that Trump stubbornly refuses to change course is another day that it becomes increasingly likely he may not only tank his own re-election bid but could be on a kamikaze mission to take the Republican-held Senate down with him. At this point, a net gain of five to seven seats for Democrats looks far more probable than the one to three seat gain that would leave them shy of a majority."