Brewer announced her choice Sunday on NBC's "Meet the Press," two days ahead of her state's Republican presidential primary.
"I think he's the man that can carry the day," Brewer said, citing Romney's business background in addition to his electability. "Mitt is by far the person that can go in and win."
Though Brewer's endorsement could help Romney woo conservatives, a group that is still notably cool toward the national front-runner, it may be more problematic for another important constituency: Latino voters, who will be an important voting bloc in November, especially across the Southwest.
Brewer's tough stance on illegal immigration and combustible rhetoric about the effects — during her 2010 reelection campaign, she made unsubstantiated claims about headless bodies turning up in the desert — make her a pariah to many Latinos.
Her backing was likely to inspire the same criticism that attended the earlier endorsement of former California Gov. Pete Wilson, who alienated many Latinos by pushing Proposition 187, the 1994 ballot measure targeting illegal immigrants.
Stephanie Cutter, a deputy campaign manager for President Obama, issued a taunting tweet on Saturday night. "Gov. Brewer endorsement tomorrow?" she said. "Great for Republican primary, damaging to general election. Say goodbye to Hispanic vote."
But Margaret Kenski, a GOP pollster in Tucson, said surveys have shown that many Latino voters support a tougher approach on illegal immigration.
That said, she dismissed the significance of endorsement, good or bad, in a high-profile race like the presidential campaign. "Everybody likes to have someone by their side," she said. "But I don't think they make that much difference."
In her comments, Brewer did not mention Romney's stance on illegal immigration, which stands to the right of his GOP opponents'. In addition to building an impenetrable fence along the U.S.-Mexico border, Romney has called for "self-deportation" of the millions of illegal immigrants in the country, a proposal that rival Newt Gingrich and others have mocked as impractical.
Arizona has become the gateway for most of the illegal immigration into the country, and in response lawmakers have adopted some of the country's harshest crackdown measures. That has led to a legal battle with the Obama administration, which is now before the Supreme Court.
In an Arizona debate last week, Romney lauded the state's approach, including a requirement that police, while enforcing other laws, question the immigration status of people they suspect of being in the U.S. illegally. Romney called Arizona a model for the rest of the country.
In backing the former Massachusetts governor, Brewer joined the rest of the state's most prominent Republicans, including Sen. John McCain.