Democrats hold a massive voter lead in states where voters register by party, a gap of 12 million that could be key to whether the party takes control of the House and Senate in the fall midterm congressional elections, according to a new analysis.

Overall, 40 percent of voters in 31 party registration states are Democrats, 29 percent are Republicans, and 28 percent are independents, according to a new report of July numbers from the University of Virginia’s Center for Politics. The states include several with key battles over House seats such as California, New York, Florida, and Pennsylvania.

In Colorado, the two parties are almost tied, with a narrow lead for Democrats.

The lead nationwide is significant, said Rhodes Cook’s analysis in Center Director Larry Sabato’s “Crystal Ball” newsletter, because in the past presidential election the majority party in 24 of the 31 states won, especially among Republican states.

In 2016, Trump won 11 of 12 majority-Republican states, and took six of 19 Democratic states, said the analysis.

Of note, some of the states with registered Democrat advantages, like Louisiana, Kentucky, and West Virginia, have been functionally Republican at the presidential level for at least 15 years, said Kyle Kondik, the managing editor of the Crystal Ball.

That could be good news for the Republicans in the upcoming election, a sign that just being majority Democrat does not mean voters are in lock step, said the report which highlighted the growth of independent voters.

But it also noted that as the nation becomes more partisan, declaring party membership is an affirmative political stand.

“With the growth in independents, many voters seem to be saying to the two major parties: ‘a pox on both your houses,'” wrote Cook.

“Yet it also can be argued that registering Democratic or Republican is far more of a statement than it once was. In the current age of sharp-edged partisanship, there is far more than a ‘dime’s worth of difference’ between the two major parties, so registering as a Democrat or Republican is a very intentional act of differentiation,” added the report.