Rosario Marin, the highest-ranking Latino woman in the Bush administration, resigned Thursday as U.S. treasurer -- clearing the way for an expected Senate campaign against Democratic incumbent Barbara Boxer.

Marin, 44, who was born in Mexico, sent a letter Thursday to Treasury Secretary John Snow that gave little insight about her decision to leave the post June 30.

"After a long and thoughtful consideration, my family and I have decided to go back home to California," she wrote.

The pro-choice moderate has already met with White House political strategist Karl Rove about a campaign against Boxer and has been talking to key Republicans about gathering support. The possibility of Marin's candidacy has excited some high-ranking GOP officials who believe she could boost President Bush's efforts to attract Latino voters.

But a candidacy by Marin -- the former mayor of Huntington Park (Los Angeles County) -- would also set up a potentially aggressive contest with another pro-choice Bush appointee -- Toni Casey, the former three-time mayor of Los Altos Hills, who's already declared her intention to run.

"With the background I've got, in world experience and education . . . I can match Barbara Boxer dollar for dollar (in the campaign), and debate her on the issues," Casey said.

"I think I differ a great deal from Rosario," she said. "She's a very nice person, but she doesn't have the depth or breath of experience I bring to the table -- nor the knowledge of the high-tech industry. . . . I think I have a much more diverse background, with my work experience from small business to big business to biotech."

Rep. Doug Ose, R-Sacramento, was considered a leading candidate to compete with Boxer until last week when he announced he would not make the run -- a move seen as a boost to Marin's chances.

Marin was unavailable for comment Thursday.

Casey had been a Bush appointee as director of intergovernmental affairs at the Small Business Administration for 18 months before she left to campaign for the GOP nomination in the March 2004 primary.

"I talked to the White House before I made the decision to run . . . had I gotten any discouragement, I wouldn't have jumped in," Casey said.

But the businesswoman is also taking shots from GOP conservatives -- about her Democratic past, donations to former President Bill Clinton, and past financial support of pro-choice causes like Emily's List.

"Let me quote one of my favorites: Ronald Reagan, who was also a Democrat" early in his career, Casey said in response. " 'I didn't leave the party. The party left me.'

"I very much believe in the conservative values of the Republicans Party," she said. "And Republicans head the list of my contributions."

Already, Casey has hired a major Republican political consulting firm, has a Web site, www. tonicaseyforsenate.com, and an A-list of Silicon Valley endorsers, including Will Corrigan, chief executive of LSI Logic; Intuit co- founder Tom LeFevre; Gordon Campbell, co-founder of Chips and Technologies; Sequoia Capital senior partner Don Valentine; venture capitalist Tim Draper; and Bette and Patrick Ng, president and CEO of Reliance Technology.

And some political analysts said Casey -- who made headlines at the 2000 Republican National Convention when she called for dropping the anti-abortion plank from the GOP platform -- is positioned to run a strong campaign.

"The real question is: Do active Republicans want to win?" said Barbara O'Connor, a professor of political communications at California State University at Sacramento. "Talk about crossover votes and women -- (Casey) is absolutely a draw. She's charismatic, has money and is a good speaker. If (the GOP) wants to make changes, she's a very viable candidate."

But others are also being talked about: former Gov. Pete Wilson -- Marin's former boss -- in addition to former gubernatorial candidate Bill Simon and Rep. George Radanovich of Mariposa.

GOP leaders say they believe that Boxer -- whose job approval ratings hover just below 50 percent -- is vulnerable, although many felt the same way in 1998 when the Bay Area Democrat won re-election.

Some GOP insiders say Marin has strongly impressed audiences and party insiders, talking up her background as a Spanish-speaking immigrant to the United States and stressing her classic American success story.

But others worry she is completely unknown to the voters and may lack the aggressive personality for a tough campaign against Boxer.

Roy Behr, Democratic consultant to Boxer, said Marin's possible entry into the race is "yet another sign of desperation on the part of the Republicans. We basically have a former city council member from the 116th largest city in California who is their best hope of defeating Barbara Boxer."

And as for Marin's possible value in attracting Latino voters, Behr said, "It's truly a measure of how out of touch the Bush White House is . . . that (they believe) getting behind Pete Wilson's former liaison to the Hispanic community is the way to reach out to Latinos in California."

E-mail Carla Marinucci at cmarinucci@sfchronicle.com.