PARK CITY, Utah — The presentation is an 11th-hour rebuttal to the fatalism permeating the Republican establishment: Slide by slide, state by state, it calculates how Donald Trump could be denied the presidential nomination. Marco Rubio wins Florida. John Kasich wins Ohio. Ted Cruz notches victories in the Midwest and Mountain West. And the results in California and other states are jumbled enough to leave Trump three dozen delegates short of the 1,237 required — forcing a contested convention in Cleveland in July. The slide show, shared with The Washington Post by two operatives advising one of a handful of...