ROSEMONT, Ill.—Rick Santorum and Ron Paul have never gotten along, and while the primaries are effectively over, their intraparty rivalry could stretch on through the summer. With 267 delegates pledged to him so far, Santorum is planning to flex his muscle at the Republican National Convention in August, where he predicted Friday there could be a showdown over the party platform between the social conservative delegates who pledged support for him and Ron Paul's libertarian supporters. Paul's campaign predicts that about 200 delegates will attend the convention on his behalf. Both want a piece of the party platform, but the...