After a rocky start to his general-election campaign, a sizable number of GOP insiders want the party to change the rules to short-circuit Donald Trump’s nomination at the July national convention. Nearly four-in-10 Republican members of The POLITICO Caucus — a panel of activists, operatives and strategists in 10 key states — would like to see changes that could deny Trump the party’s nod after the presumptive nominee began the November campaign without a credible campaign structure, in addition to making a series of erratic and inflammatory comments. Among the comments was a recent suggestion that an American-born judge presiding...