Yes, it can do. Without the real threat of UKIP taking votes from the Conservative Party in the UK, David Cameron would not have promised the in/out referendum on EU membership.
Or put more simply: his personal standing within the party was threatened more than his party's electoral chances (UKIP votes were irrelevant in most Tory seats and took votes off Labour too in marginals), and he wouldn't have been in power anyway under a proportional system.
Ironically, EU Parliament votes being proportional probably played a major role in Farage deciding back in the nineties that he could try to win the Brexit debate from outside the Tory party too...