SAN JUAN, Puerto Rico — The plodding Republican presidential nomination fight is grinding forward toward Puerto Rico — and a two-man race, with Rick Santorum ascendant and Mitt Romney vanquished in the Deep South. Former House Speaker Newt Gingrich, the former Georgia lawmaker whose Southern strategy stalled, was all but relegated to an asterisk in the contest even as he vowed to stay in it. “Now is the time to pull together,” Santorum declared to conservatives in Lafayette, La., after winning Tuesday’s primaries in Alabama and Mississippi, urging the party’s faithful to unite behind him to beat Romney. “We are...