Some figures....
There are 35 seats up for re-election in 2020. Currently, they are split 12 for Democrats and 23 for Republicans. To gain control of the Senate, the Democrats will need to win 17 of the 35 up, taking 5 from the Republicans. This would give them a 51-49 majority. In case of a tie in a vote, the VP casts the deciding vote.
There are currently four seats that are considered a toss-up in this election, in Maine, North Carolina, Arizona and Colorado. 1 is a Democrat seat and 3 are Republican.
There are an additional 4 Republican seats that are leaning Republican but are not safe and 1 Democrat seat that's leaning Democrat but isn't safe.
It won't be easy to flip the Senate but now's the time.