The point is that center right Republicans (the kind that used to win most Republican districts before 2012) could still win if they could make it to the general election. They often can't because most Republican voters, like most Democrat voters, aren't into party politics enough to bother voting in the primaries.
It is the voters who are most likely to be to be farthest from the center who vote in the primaries, and these are the ones who don't want a normal center right representative.
If Democrats switched parties and voted in the primaries they might be able to counter the usual extreme primary voters so a center right Republican could win.
The point is that center right Republicans (the kind that used to win most Republican districts before 2012) could still win if they could make it to the general election. They often can't because most Republican voters, like most Democrat voters, aren't into party politics enough to bother voting in the primaries.
It is the voters who are most likely to be to be farthest from the center who vote in the primaries, and these are the ones who don't want a normal center right representative.
If Democrats switched parties and voted in the primaries they might be able to counter the usual extreme primary voters so a center right Republican could win.