Political analyst David Shor recently made an (off the cuff) comment implying that an additional ~800,000 votes would be sufficient for Democrats to win back the Senate. It's unclear whether he meant win it back with a 90% probability or a 99% probability, hence this Metaculus question, generalized to ask how close the elections for Senate control will be.
By how many votes will the losing party fall short of winning control of the Senate in the 2020 Senate elections?
The number of votes by which the losing party falls short of winning control of the Senate will be determined by summing the number of votes in each of the closest state Senate elections (measured in absolute number of votes) necessary for that Party to have won the election, based on the best available published estimates of the margin of victory in each state Senate election.
If the Republican Party retains control of the Senate after the 2020 elections, this question will resolve as the number of votes the Democratic Party fell short by; if the Democratic Party wins control of the Senate it will resolve as the number of votes the Republican Party fell short by.
For details on how party affiliation and how Senate majority are determined, this question will resolve according to the same detailed criteria given under "See the Full Rules" on the PredictIt question Which party will control the Senate after 2020 election?. That is, in the case of an apparent tie in the Senate, the majority party will be considered to be the party that is the same party as the Vice President. Similarly, a Senator's party affiliation shall be determined by whichever party’s caucus she or he is a member of following the election or would have been a member of had he or she been elected (meaning that e.g. Independent Party Senators may be considered to contribute to either the Democratic or Republican Party's numbers depending on which party's caucus they are a member of).
Since the Vice President fits into the definition of which party controls the Senate above, if it is the case that the margin of victory that the Vice President won by (measured in absolute number of votes made by regular voters, not by presidential electors) is low enough that changing that outcome would be the most efficient way to change which party controls the Senate, then the margins of victory for the 2020 general election state popular votes may be factored into the determination of the number of votes by which the losing party falls short by.