The clash over an Arizona ballot measure designed to crack down on illegal immigrants echoes the political debate over immigration that has played out across the American Southwest for four decades.

There is, however, one notable departure from the script: The Republican Party appears to have switched sides.

“The change is almost hard to describe it’s so enormous,” a prominent Hispanic activist, Cecilia Munoz of the National Council of La Raza, said.

“This initiative in particular has been disavowed by the Republican Party leadership in Arizona,” an executive vice president of the Service Employees International Union, Eliseo Medina, declared. “It’s been stripped of any of its respectability.”

The Arizona initiative that has prompted the GOP’s about-face is a ballot measure that would require state and local officials to verify the citizenship of all who seek government services. Proposition 200, as it is known, also tightens safeguards on voting, requiring proof of American citizenship from those registering to vote, and some form of identification from voters at polling places. In addition, the initiative would require state and local employees to report suspected undocumented immigrants to federal authorities.

The drive for the proposal on Tuesday’s ballot has been directed largely by a grassroots movement of local activists, with some financial backing from national organizations that favor tighter immigration controls.

An impressive and diverse coalition has joined forces to fight the proposition. Opponents include business groups, unions, health-care providers, and politicians from across the political spectrum, including Governor Napolitano, a Democrat, as well as the state’s two senators, Jon Kyl and John McCain, who are Republicans.

Opponents of Proposition 200 say it could discourage ambulance crews and doctors from treating patients who appear to be Hispanic. Critics also say the measure would be costly to local governments, which might have to verify that all those who get garbage pickup or water service are citizens or legal residents.

“I understand the frustration of Arizonans, especially southern Arizonans,” Mr. McCain said earlier this month. “Things are terrible, and we’ve got to fix it. But we’re not going to fix it until we have comprehensive immigration reform.”

Supporters of the initiative say it will have no impact on the provision of emergency medical treatment and could save taxpayers money by weeding out noncitizens from government programs.

Any discussion of the Arizona measure quickly turns to a similar ballot initiative put forward in California a decade ago, Proposition 187. The then governor of the state, Pete Wilson, led the charge to approve the immigration crackdown measure, which was more far-ranging than the proposition Arizonans are to vote on next week.

Anger over the costs of providing education, medical care, and other social services to illegal immigrants helped carry Mr. Wilson to re-election in 1994, but many observers now view his decision to back Proposition 187 as a colossal mistake.

“You can win the battle but still lose the war. The Republican Party has never been the same in California again,” Mr. Medina said.

Political analysts said the GOP’s support for Proposition 187, along with subsequent measures that sought to limit bilingual education and racial preferences, drove black and Hispanic voters away from the party and even stunted the party’s inroads among young Anglos.

“The California lesson is this is a very dangerous game for the Republican Party to play,” said a professor of political science at the University of California at Riverside, Shaun Bowler.

“It’s often quite good short-term politics. It’s never good long-term politics, and Pete Wilson is the poster child for that,” Ms. Munoz said. Mr. Wilson, once viewed as a presidential contender, lost to Gray Davis in a bid for re-election in 1998.

Proponents of Proposition 200 in Arizona said they feel betrayed by the prominent Republicans who are publicly opposing the measure.

“Am I disappointed? Absolutely,” said a Republican state legislator from Mesa, Russell Pearce. “I see that as the elite. I see that as politicians not listening to the voters again. Poll after poll after poll, Americans want to see their borders secured and the law enforced.”

Surveys have shown broad backing for Proposition 200, although the level of support seems to be on the decline. In July, a whopping 74% of voters polled said they favored the measure. A poll released on Friday by the Arizona Republic pegged support for the initiative at 55%.

One earlier survey showed support for the measure dropping as low as 42%, but news coverage of a gory traffic accident may have pushed the numbers back up. On October 16, a truck suspected of carrying illegal immigrants flipped over as it sped away from police near Fort Huachuca in southeastern Arizona. The truck set off an 11-vehicle pile-up that killed six people and injured 15 others.

Neither President Bush nor the Democratic presidential nominee, Senator Kerry of Massachusetts, has staked out a clear position on Proposition 200. Many attribute that to a desire on the part of both men to do nothing that might alienate swing voters in a contested state. Arizona is said to be leaning toward Mr. Bush, but is not quite out of Mr. Kerry’s reach. The most recent public poll, taken by the Arizona Republic newspaper, had Mr. Bush at 47% and Mr. Kerry at 40%.

“The early polls said 60% or 75% of the voters in the state were for it. Who wants to kick them in the teeth?” said a policy analyst with the Manhattan Institute, Tamar Jacoby.

She said, however, that she believes Mr. Bush has signaled his opposition to the measure. Earlier this month, the president’s communications director, Daniel Bartlett, told reporters the president favors tight border controls, but “wants to have a more humane system of treating illegal immigrants who are here.”

A professor of political science at the University of Arizona, John Garcia, noted that Mr. Bush throttled an anti-immigration effort earlier in his political career. “There were similar rumblings in Texas when George Bush was governor. The Republican Party in Texas squelched that quickly,” Mr. Garcia said.

A spokeswoman for Mr. Kerry, Susan Walitsky, said the senator views the proposition as a state issue. “However, Senator Kerry is clear he would be against anything that impedes a person’s ability to participate and vote,” she said, “as well as anything that impedes a person’s ability to get lifesaving services.”

California passed Proposition 187 with 59% of the vote, but most of its provisions never took effect, because of a series of court challenges and injunctions. If Proposition 200 passes in Arizona, opponents are certain to bring similar litigation. One fertile line of attack may be a printing error involving petitions circulated to put the proposition on the ballot. Some petitions referred to denying “public welfare benefits” to illegal aliens, but the word “welfare” does not appear in the official language certified for the election.

A former congressman from Arizona, Matthew Salmon, said he thinks the GOP’s concerns about alienating Hispanic voters are misplaced. “I think they’re making a miscalculation,” he said. Mr. Salmon, who supports Proposition 200, noted that some polls have shown that more than 50% of Hispanic voters back the measure.” A lot of those are people who waited in line, did it the right way,” he said. “They have a problem with people crossing the border and doing it illegally.”

The GOP leadership is far from united in its response to the ballot measure. Mr. Pearce said more than half the Republican state legislators support it, as do more than 75% of precinct committeemen across the state.

While opponents of Proposition 200 said Arizona’s delegation in Washington has unanimously rejected the measure, that may be untrue. According to Mr. Salmon, Rep. Trent Franks, a Republican from Glendale, is supporting the initiative.” He left me a voicemail message saying,’ I’m voting for it, though I have a couple of concerns,'” Mr. Salmon told The New York Sun yesterday.

Advocates for Proposition 200 listed Mr. Franks as a planned speaker at a rally they held last week, but he did not show up. An aide to Mr. Franks, Lisa Teschler, said that until the issue about the measure’s wording was resolved, the congressman “didn’t want to comment on it one way or the other.”

The union leader, Mr. Medina, maintains it is untenable to be seen as anti-immigrant in a state with a burgeoning Hispanic population. He credits GOP leaders for standing up to the anti-immigration elements in their ranks. “The Republicans are saying that the tide of history and democracy is not with that wing of the party,” Mr. Medina said.

He acknowledged that some of those opposing Proposition 200, such as chambers of commerce, are not traditional allies of the labor movement. “Sometimes politics does make for strange bedfellows,” he said.