The Detroit Free Press declared Romney the victor over his GOP rivals in Michigan.
With about 96 percent of the precincts counted, Romney had 390,879 votes (41 percent) to Rick Santorum's 360,769 (38 percent), results posted by the Free Press showed. Ron Paul was third with 110,948 votes (12 percent) and Newt Gingrich followed with 62,102 votes (7 percent). Another 2 percent voted "uncommitted" and a handful of votes went to several other Republicans who are no longer contenders.
CNN called Romney the winner in Arizona. With 79 percent of the vote counted, Romney had 208,999 votes (47 percent) to Santorum's 115,687 votes (26 percent). Gingrich was third with 71,633 votes (16 percent), with Paul bringing up the rear with 37,314 votes (8 percent).
Santorum took solace in not getting blown out by Romney and called it "an absolutely great night."
"This was going to be Romney's night. The question was how big. And it wasn't very big," CNN quoted Santorum as saying.
"A month ago they didn't know who we are, but they do now."
Back in the GOP driver's seat, Romney said the margin of his twin victories didn't matter.
"A week ago, it was just a week ago, the pundits and the pollsters, they were ready to count us out," he said. " ... We didn't win by a lot, but we won by enough and that's all that counts.
Exit polls in the two states Tuesday had indicated electability was the top priority among Republican primary voters.
Thirty-three percent of Michigan voters said beating President Obama is the top priority when selecting a GOP nominee, while 38 percent of voters in Arizona said the same, CBS News exit polls indicated.
Romney and Santorum slashed at each other ahead of the Michigan primary, which was too close to call. The contest in his home state is seen as crucial to Romney and his claim of the front-runner's mantle.
Despite the closeness of the Michigan race, the Post said Romney was expected to win about three times as many convention delegates as Santorum Tuesday. Arizona is a winner-take-all primary. Michigan's 30 delegates are allotted proportionately to the primary winner in each of the state's 14 congressional districts. Arizona's winner picks up all of its 29 delegates.
The Washington Post said Romney was expected to garner 45 delegates from both contests, with Santorum picking up about 15.
Gingrich and Paul were not expected to win any delegates, and did not seriously campaign in the two states.
Romney, the former Massachusetts governor, and Santorum, a former U.S. senator from Pennsylvania, have been shooting fireballs at each other in recent days.
Romney blasted Santorum's use of robocalls urging Democrats, who can vote GOP in Michigan, to vote for Santorum.
"I know why [President] Obama doesn't want me to face him but I just think it's outrageous and a terrible dirty trick at the last hour, by the way, late in the afternoon on the day before the election, maybe hoping no one would notice, they start sending out calls to Democrats, union members telling them to go into the Republican primary and vote against Mitt Romney," Romney told Fox News.
"This is a new low for his campaign and that's saying something," he added.
Santorum told Fox News Monday night he's just trying to attract the Democratic voters he'll need in November.
"When he runs a robocall of my voice from four years ago saying good things about him, that's not a low moment, and when I run a call basically saying, calling Democrats that are eligible to vote here, to vote for us, that's a low?" he said.