More than a dozen former Ronald Reagan staff members have joined dozens of other Republican figures endorsing the Democratic nominee and vice-president, Kamala Harris, saying their support was “less about supporting the Democratic party and more about our resounding support for democracy”.
You are talking about long term consequences. Like capitalists who only look to the current quarter’s profits, politicians only look to the current election.
A smaller voter base is easier to appeal to. Both parties have relatively small voter bases and no competition because of the first past the post system and the electoral college.