If they are big enough to ruin re-election chances then they are by definition big enough to make pursuing their votes worthwhile (because without them you will loose)
If you want a system where you can disregard that reality then you need a different electoral process. An easy way to mitigate that risk is to eliminate 3rd party candidates and make voting mandatory (or pressure 3rd parties to drop out and guilt non-voters into voting, as it were), but an astute observer might notice that looks an awful lot like something called a ‘sham democracy’.