Two other Republican candidates for president, 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.
Romney won the state with 41 percent to Santorum's 38 percent. Paul had 12 percent of the vote to 7 percent for Gingrich.
In Ottawa County, with all 109 precincts reported, Santorum finished with 48.7 percent of the GOP vote. Romney took 35.5 percent, Paul had 8.5 percent and Gingrich had 4.7 percent of the Republican ballots in the county, where the turnout was 24.8 percent of the registered voters.
Santorum also topped Romney in Muskegon and Kent counties.
Allendale Township zoning referendum (5 of 5 precincts reported):
Yes 1,093 (43.2%)
No 1,439 (56.8%)
Olive Township Fire-Emergency millage renewal (2 of 2 precincts reported):
Yes 557 (73.2%)
No 204 (26.8%)
Zeeland Public Schools operating millage (21 of 21 precincts reported):
Yes 4,362 (68.1%)
No 2,046 (31.9%)
A school millage in the Ravenna district of Muskegon County was defeated by a single vote, 335-334.
The Associated Press contributed to this report.