From the Cook Report, with 100 days to the election, Democrats are favored to take control of the Senate.
The current composition of the Senate is 53 Republicans, 47 Democrats (includes 2 Independents who caucus with the Democrats). In order to win the Senate, the Democrats would need to pick up 4 seats, or 3 seats plus the Presidency. There is an expectation that Doug Jones of Alabama loses his seat, so in order to pick up the ~5 seats Democrats need.
It Appears that Arizona and Colorado will elect Democrats; Mark Kelly is an extremely strong candidate running against an appointed Senator (she lost her race to the Senate) and polls show Kelly with a 7-10% lead. Colorado is a state that Clinton won, and the state seems to have moved left in the intervening 4 years; there have been few recent polls but Hickenlooper had a double-digit lead against the Incumbent since he won the primary.
North Carolina and Iowa are both showing a Democrat lead. Montana has a popular former governor running for the Senate with a virtual tie with the incumbent; although it is a ruby-red state, it looks like it will a close election. Susan Collins in Maine is very vulnerable, polling behind her Democratic challenger Sara Gideon.
The most vulnerable Democrat is in Michigan - where polls show Democratic incumbent Gary Peters leading John James by 9%.
Summary: Democrats are defending 12 seats is 'solid D' territory, 1 in Lean D territory, and one in Lean R territory that they're likely to lose. Pickup opportunities for the Democrats include Arizona (rated lean-D), Colorado, Ga (1 of the 2), Iowa, Maine, Montana and North Carolina. Even in 'Lean Republican' territory (Georgia's OTHER election and Kansas) there are signs that it may end up being more competitive.