When internal migration favors red states, that helps Republicans in the short term even if the people who are moving are more progressive in their personal politics than the communities they’ll be entering. Most obviously, red states will gain Electoral College votes and extra congressional seats after the next census, and blue states will potentially lose them.
But, over time, it can change our politics profoundly. We’ve seen growth in northern Virginia turn that state from red to purple to blue. North Carolina is a swing state now. Georgia is getting close to that point. And, Texas will be purple by 2028 at the latest. In fact, the Democrats are already targeting Texas congressional districts, the state was closer than Iowa (previously a swing state) in the last election, and polls have come out showing that Sen. Ted Cruz may be an underdog in his reelection effort.
Needless to say, if Texas turns purple it will sap the Republicans of funds. And if it turns blue, they’ll need to capture the whole Upper Midwest to compensate or they’ll be out of luck in presidential contests.