Polling showed that had McCain run Reform Party in 2000, he'd have started at 24% versus Gore's 34% and Bush's 35%.
McCain ruled it out in March of 2000, but Bush's campaign was incredibly dirty and McCain advisers were a bit more open to the idea when pressed.
What if McCain opted to run Reform?
Angust King would be a strong running mate. Other prospects might be Lowell Weicker or Dick Lamm. I'd say Lieberman too, but he had reelection that year so it'd be tough.