WASHINGTON — Mitt Romney coasted to victory in the Arizona primary Tuesday night and vied with rival Rick Santorum for supremacy in Michigan in a Republican presidential race as unsettled as the day it began. Two other candidates, Newt Gingrich and Ron Paul, made little effort in either state, pointing instead to next week’s 10-state collection of Super Tuesday primaries and caucuses. Romney’s Arizona triumph came in a race that was scarcely contested, and he pocketed all of the 29 Republican National Convention delegates at stake. Michigan was as different as could be — a hard-fought and expensive race in Romney’s home state that he could ill afford to lose and Santorum made every effort to win. Returns from 13 percent of Michigan’s precincts showed Romney at 41 percent and Santorum at 38 percent. Paul was winning 11 percent of the vote to 7 percent for Gingrich.