Nuclear freeze. With Reagan as POTUS in OTL, Kennedy pushed for a nuclear freeze and blamed Reagan's nuclear build-up (which actually started under Carter) for increased tensions.
As I posted in the 'Better USSR' thread, US really didn't have an increase in overall weapon numbers, but with more capable and survivable weapons.
Reagan's real legacy was with continuing Carter's Conventional buildup on land, and for really upsizing the USN