Democrats need 50 seats to control the U.S. Senate if Biden wins the Presidential race (a net gain of three seats), and 51 if Trump wins the Presidential race (a net gain of four seats).
It is more likely than not that Democrats will regain control of the U.S. Senate, and Biden has a clear lead over Trump in Presidential polling at the moment, so they may need a net gain of only three seats. But it is far from certain.
Michigan, Minnesota, New Hampshire and New Mexico have Democrats running for re-election in the U.S. Senate this year, all of whom are likely to win re-election, although the races in Michigan and Minnesota have polling that favors the Democratic incumbents by small enough margins that the races could be lost.
Democrats are almost sure to lose a U.S. Senate seat in Alabama, where incumbent Democrat Doug Jones, trails his Republican challenger, Tommy Tuberville, by 10-17 percentage points in the polls.
This means that the Democrats need to pick up 4 more seats to control the Senate if Biden wins, and 5 more seats to control the Senate is Biden loses.
Democrats are leading in 4 races that could produce turnovers, have 3 more potential turnover races that are toss up races, and have a realistic underdog's shot at 3 more races.
Which turnover seats are in play:
* Arizona, where Kelly, the Democrat, leads by healthy margins in the lion's share of recent polls in a race against the Republican who filled John McCain's vacancy.
* Colorado, where Hickenlooper, the Democrat leads by healthy margins over incumbent Republican Cory Gardner in all recent polling, despite a lackluster primary campaign.
* Maine, where Gideon, the Democrat leads incumbent Republican Collins by a modest by stable margin in almost all recent polling.
* North Carolina, where Cal Cunningham, the Democrat narrowly leads incumbent Thom Tillis in the lion's share of recent polls by a thin to moderate margin.
* Iowa, were Greenfield, the Democrat, leads the Republican, Ernst, by a thin margin in most recent polls, but trails Ernst by one percentage point in the highest quality recent poll.
* In Georgia, there are two U.S. Senate races. In one, the Democrat, Ossoff faces the Republican Perdue, in a race that is a polling toss up. In the other, the Democrat, Warnock, trails the Republican, Loeffler and Collins, consistently, in the polls.
* In Kansas, the race between Bollier, the Democrat, and Marshall, the Republican, is a toss up in the polls.
* In Alaska, the Democrat, Gross trails the Republican, Sullivan, in most polls, but did tie Sullivan in the most recent poll.
* In Kentucky, the Democrat, McGrath, trails incumbent Senate Majority Leader Mitch McConnell, but his lead had shrunk to the low single digits in many recent polls.
* In Montana, the Democrat, Bullock, had been leading in the polls in early July, but the Republican, Daines, has had a modest lead in the polls since then.
Earlier hints that the U.S. Senate races in Texas might be winnable for the Democrats are not supported by any recent polling.
Real Clear Politics, when push comes to shove, sees Democrats gaining a majority in the U.S. Senate in this election, although their call of Colorado as a "toss up" because they don't have any polling in the race (even though 538 has plenty of polls linked above) is laughable.