COLUMBIA, S.C. — Mitt Romney collected the endorsements Wednesday of the architect of Arizona’s immigration-crackdown law, marking the final step on a journey that has taken him from lukewarm support of legalization to the Republican presidential field’s most ardent opponent of amnesty.

And with Mr. Romney inching closer to wrapping up the GOP’s nomination, it sets up what would be the strongest contrast ever between the two major parties’ candidates on immigration.

“Romney stands head and shoulders above the crowd,” Kris W. Kobach, Kansas’ secretary of state and the architect of the Arizona law, told The Washington Times, praising Mr. Romney for treading where other Republican candidates have refused to go. “Immigration is one of those issues that will appear to be a hot button — some elected officials who are afraid of offending anyone will avoid taking tough stands on immigration, and he took a tough stand.”

Mr. Kobach’s endorsement follows those of other leaders in the immigration crackdown movement, including Bay Buchanan, who ran Tom Tancredo’s presidential campaign in 2008 and who is the sister of former presidential candidate Patrick J. Buchanan.

It’s no surprise that the issue is popping up with 10 days to go before Republicans vote in South Carolina’s primary. South Carolina has passed its own Arizona-style law seeking to make illegal immigration a crime, giving state and local authorities police powers, and Republican primary voters in the state are overwhelmingly in favor of it.

The Obama administration has sued to halt the South Carolina and Arizona laws, as well as similar laws in Utah and Alabama, and those cases are winding their way through federal courts.

Mr. Romney and others in the Republican presidential field have said they support the states’ efforts, and Newt Gingrich on Wednesday told South Carolina voters that one of his first actions as president would be to drop the lawsuits.

Still, those who favor a crackdown on illegal immigration say Mr. Romney has emerged as their clear favorite over the past few years. He has embraced the use of E-Verify, the government’s voluntary system for checking work status; has talked about attrition of the numbers of illegal immigrants in the country through enforcement of laws; has called for building more border fencing; and has embraced state efforts to pick up enforcement where the federal government has failed.

Most of all, he adamantly vowed to block legalization bills. He recently said he would veto the Dream Act, which would grant legal status to illegal-immigrant students and young adults, who are usually considered the toughest cases because many of them were brought to the U.S. by parents and have never known any other country.

“He has held the line 100 percent on this amnesty stuff,” said Roy Beck, executive director of NumbersUSA, which grades the candidates on the issue.

Immigrant rights advocates said all of that makes for a stark contrast with Mr. Obama, and will make it tough for Mr. Romney to win Hispanic votes in November.

“Honestly, the Obama campaign must be doing high-fives. This guy is handing them the Hispanic vote when Obama is vulnerable with Hispanic voters,” said Frank Sharry, founder of immigrant rights advocacy group America’s Voice. “To me, it’s political malfeasance. He should be charged and found guilty by a jury of his peers.”

Mr. Sharry said Mr. Romney has changed on this issue. Before his first presidential campaign, he complimented President George W. Bush’s legalization plans and praised Sen. John McCain of Arizona, who led the Senate fight for legalization.

But by the end of his time as governor of Massachusetts, Mr. Romney had vetoed a bill offering in-state tuition rates to illegal immigrants and signed an agreement with federal authorities that would have allowed state and local police to assist in immigration enforcement. In the 2008 campaign, he ran ads in Iowa attacking Mr. McCain on immigration.

In between that campaign and this one, Mr. Sharry said, Mr. Romney’s strategists reconsidered and decided to de-emphasize stances on abortion, gay marriage and immigration.

“What’s remarkable is they followed the playbook on abortion and gay marriage, but not immigration. I am, just from a political point of view, flabbergasted that he has run so far to the right on this issue, and what that will do to his electability in the general election,” he said. “And so what? So he can be more conservative than Rick Perry and Newt Gingrich? As if it’s the defining issue of the conservative base? It’s not.”

All of that makes Mr. Romney the most vociferous opponent of illegal immigration in the field.

The contrast between the former governor and Mr. Obama was underscored this week when Mr. Romney trumpeted the endorsement of Mr. Kobach, while at the White House Mr. Obama gave a promotion to Cecilia Munoz, a chief backer of the legalization bill Mr. Bush and Sen. Edward M. Kennedy, Massachusetts Democrat, tried to push through the Senate in 2007. Ms. Munoz was at the National Council of La Raza at the time, left that group to join the White House in 2009, and on Monday was promoted to run the White House’s Domestic Policy Council.

“It just goes to show that Obama is trying to close down his vulnerability with Hispanic voters, and Romney is opening up his,” Mr. Sharry said.

After Mr. Romney’s promise to veto the Dream Act, students have begun to follow him and challenge his position.

In an interview with The Times this weekend, Mr. Romney didn’t back down, saying he doesn’t see much room for exceptions to his policy that illegal immigrants should go home.

“I will put out a series of immigration proposals before November of 2012, and I will look at adjustments to the law that I think maybe necessary, but my principle is straightforward and that is that those who’ve come here illegally really should not be given a preferential path to permanent residency or citizenship,” he said.

Hispanic voters are considered a critical swing group in the upcoming election, and Mr. Romney has said he needs to do well in attracting them.

The Republican National Committee also is committed to the effort, and on Wednesday it announced its Hispanic voter-outreach program, calling Hispanics “essential” to the GOP’s 2012 election.

One early test for the GOP and Hispanic voters will come Jan. 31 when Florida holds its primary. Mr. Romney on Wednesday announced a Spanish-language ad he is running in that state, featuring his son Craig and three Cuban-American Republicans, two of whom are in Congress and the other of whom is a former member.

• Seth McLaughlin contributed to this article from New Hampshire.

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