California.
That answer may strike some people as appalling.
It is not.
This would be a longterm strategy should it play out that the Democrats flip and carry Texas—and that Texas would generally trend for the Democrats.
If that were to become the case, and this would be dealing with the topic of realignment, Republicans wanting a future for their political party—with winning the presidency of the United States—would have to counter-realign and become relevant again in, and pursue, California. (And other states similar in voting pattern to California.)
You can’t have a political party be viable for the presidency of the United States while the other of the two major political parties carries the Top 2-ranked populous states in the nation. (The Top 2, nowadays, are California and Texas.)
So, right now, to answer California seems unthinkable in the minds of plenty. But, during the early part of the 2000s, people were dismissing the notion of Democrats being able to carry Virginia. During the early part of the 2010s, people were dismissing the notion of the Republicans being able to carry Michigan. (Those are just two examples.)
lol, is this a joke? The trends that are making Texas purple are the same ones keeping CA safe Dem.