But the stat means basically nothing. Or, if we're being extremely charitable, it's highly, highly misleading.
The stat appears to originate with a USA Today report on Nov. 10, when Senate Democrats were at 45.2 million votes and Republicans were at 39.3 million. That 6-million-vote margin has actually grown since then, for reasons we'll discuss.
But Democrats owe almost that entire advantage to one massive, fluke-y state. The biggest state in the union — California — just happened to be holding a Senate contest on Election Day that was a race between two Democrats. (The state has a unique “top-two” primary system.) That meant the 11.6 million votes counted there so far have all been ballots cast for Democrats. That's basically 1 out of every 8 Senate votes nationwide handed to the Democrats at the outset. (At the time of the USA Today report, only 8 million votes had been counted, so Democrats' Senate popular-vote advantage has actually grown by millions since then.)
In a normal Democrat-vs.-Republican election in California, the Democrat might have gotten 7 million votes and the Republican 4.6 million votes — a margin of 2.4 million votes. In this case, the margin was 11.6 million, a 9-million-or-so vote swing, which basically accounts for Democrats' entire advantage in the nationwide Senate popular vote.
Another factor in all of this: Only about two-thirds of the country holds Senate races in any given election, meaning that the lean of the popular vote is very reliant upon which states are up that particular cycle.
This year, the two biggest, bluest states in the country — California and New York — happened to be up. The biggest GOP-leaning state, Texas, was not. Democrats netted 11.6 million votes in California and nearly 3 million in New York, where Sen. Charles Schumer (D) won in a walk, 67 percent to 26 percent.
By contrast, Republicans won by 1 million votes in only one state — Ohio — not because they didn't do well, mind you, but because basically none of the red states were big enough to give the party such a wide popular-vote win.
This argument is especially attractive to Democrats, of course, because their presidential nominee, Hillary Clinton, also won the popular vote against Donald Trump. But it traces back to the 2012 election, in which Democrats cried foul because they won more votes for the U.S. House but still held a significant minority of the seats. In that case, some Democrats held it up as evidence that the map was too heavily gerrymandered in Republicans' favor and, accordingly, anti-democratic. But while the map is definitely drawn in the GOP's favor thanks to Republicans having won unprecedented control over state legislatures and the redistricting process in the 2010 election, their ability to win more seats than the popular vote suggests they “should” is also tied to Americans' tendency to self-sort — that is, Democrats packed into densely Democratic, urban areas, and Republicans more spread out and having a natural advantage in most of the country's congressional districts. Even “fair” districts would create a House map that favors the GOP.
The House math was also thrown off by the fact that many members of Congress don't even face an opponent, rendering the popular vote statistic less than met the eye for the country as a whole.
Philip Bump made this chart after the election:
As a talking point, the Senate popular vote should be retired immediately.