They can only win with a conservative, yes - but sometimes the Democrats go hard left and allow a semi conservative to win - as in 2000 and 2004, where a somewhat conservative Bush beat hard core radicals.
The key is not only conservatism, but how big the chasm is between the parties. When it’s big, the GOP always wins. When it’s small, the Dems always win. Thus the magic of Clinton, who was able to obfuscate any ideological differences running against moderates HWBush and Dole.
Now on HW Bush, he won in a landslide, but that was on the assumption, or hope, that it would be another 4 years of Reagan. When that was demonstrated NOT to be the case, HW Bush got only 38% of the vote in 1992.