Senator Ted Cruz’s supporters are mounting an effort to seize control of the Republican platform and the rules governing the party’s July convention, the first indication that Mr. Cruz will not simply hand his delegates over to Donald J. Trump. In an email sent Sunday to pro-Cruz convention delegates, a top aide to the Texas senator wrote that it was “still possible to advance a conservative agenda at the convention.” “To do that, it is imperative that we fill the Rules and Platform Committees with strong conservative voices like yours,” wrote Ken Cuccinelli, who was the campaign’s former delegate wrangler...