Not that different. Look at the landslide of '72 between a moderate Reublican and a liberal Democrat. Only a Dem centrist or one who portrays himself as a centrist has a real shot at the White House.
I dont think Kennedy v Reagan is moderate GOP v Liberal Dems.... Reagan was pretty radically conservative for the late 1970's, and paradoxically Kennedy was more moderate... dont get me wrong Reagan was a decent guy... but in 1980 Kennedy v Reagan is more like generic liberal democrat v conservative republican... in other words very polarised and in the end very close IMHO....