Pretty sure it only affects the primaries, not the main elections. That voter affiliation on the card doesn't force someone to vote for the GOP presidential candidate or Senate candidate on Election Day. A GOP registration simply prevents them from voting in the Democrat primary, and vice versa. If changes to registration are done without consent, it's surely illegal (here in Australia, we'd call it branch stacking, and that has ruined political careers here, and even ultimately installed a prime minister who should never have been a candidate for his electoral division), but I have no idea what sort of enforcement could be done about it in America.