Mr. Romney had to win in Michigan, basically his home state, where his father was governor. Although his margin of victory over Rick Santorum was a narrow 3 percentage points, he was able to get past the barriers to victory and made his party's nomination more likely. The first fence to jump was his opposition to President Barack Obama's auto industry bailout, a major Michigan vote-loser. The second was the overall resistance of Republican voters to his image as a less-than-fervent social conservative, in spite of his professions to the contrary.
Mr. Romney's victory in Arizona, by nearly 21 percentage points, giving him all of that state's delegates to the Republican national convention, can also be considered an affirmation of support for him by conservatives, given Arizonans' sometimes extreme positions.
The outcomes in Arizona and Michigan could mean that Mr. Romney has fought off the most recent challenge to his Republican front-runner status, this time from former Pennsylvania Sen. Santorum, rejected by voters here in 2006 by a stunning 18 percent. Mr. Santorum's having peaked, prior to a decline, would replicate the track of previous Republican candidates Donald Trump, Herman Cain, Michele Bachmann and Rick Perry, who flagged as voters took a closer look at them.
Strong performance in the races next Tuesday -- Super Tuesday -- could ice the nomination for Mr. Romney.