Republicans hold the Senate 53-47. (There are two independents — Angus King of Maine and Bernie Sanders of Vermont — but they caucus with Democrats and therefore should be counted that way in the math for Senate control.)
To flip the Senate, Democrats would need to net-gain four seats outright or three seats and control of the White House, because in a 50-50 Senate — which is possible this year — the vice president breaks the tie. Republicans can lose up to three seats and hold the majority, as long as President Trump wins reelection.