WASHINGTON — Senator Obama is targeting Senator McCain over his vacillation on immigration, telling a Latino advocacy group that the presumptive Republican nominee had “abandoned his courageous stance” on a comprehensive bill that failed in the Senate last year.

Both candidates addressed the League of United Latin American Citizens here yesterday as they court a constituency that could prove decisive in key swing states. Each pledged a commitment to an immigration overhaul, with Mr. Obama insisting he would pass legislation by the end of his first term in office.

The two rivals supported the far-reaching bill last year that would have provided a path to legalization for the nation’s estimated 12 million illegal immigrants, but Mr. McCain backed away from the measure after it stalled for the second time in as many years and nearly derailed his campaign for the Republican nomination.

After saying he “admired” Mr. McCain’s initial leadership on the issue, Mr. Obama chastised him for the election year shift. “When he was running for his party’s nomination, he abandoned his courageous stance and said he wouldn’t even support his own legislation if it came up for a vote,” the Illinois senator said. “We need a president who isn’t going to walk away from something as important as comprehensive reform when it becomes politically unpopular.”

Mr. McCain, speaking earlier in the day, largely steered clear of criticizing the presumptive Democratic nominee, and instead he paid tribute to the contributions of Hispanic-Americans and reiterated his support for broad-based immigration reform.

The country, he said, “would be the poorer were we deprived of the patriotism, industry, and decency of those millions of Americans whose families came here from other countries in our hemisphere.”

Mr. McCain recalled the unsuccessful reform effort in Congress, acknowledging the uproar it generated from conservatives, who said a path to citizenship for illegal immigrants was tantamount to amnesty.

The Arizona senator is now walking a political tightrope on the issue, standing by his support for a path to legal status and a guest worker program while endorsing the view of conservatives that border enforcement must come first.

“Many Americans, with good cause, did not believe us when we said we would secure our borders, and so we failed in our efforts,” Mr. McCain told the league yesterday. “We must prove to them that we can and will secure our borders first, while respecting the dignity and rights of citizens and legal residents of the United States.”

Before he spoke, Mr. McCain was praised by LULAC leaders, who have made passage of comprehensive immigration legislation a priority. Mr. Obama was introduced by the mayor of Los Angeles, Antonio Villaraigosa, who had supported Senator Clinton in the Democratic primary but delivered a full-throated endorsement yesterday.

Polls in recent months have given Mr. Obama as much as a 30-point lead among Latino voters, despite his struggles with the constituency during the campaign against Mrs. Clinton.

The gap appeared evident at the convention yesterday, where Mr. McCain drew polite but unenthusiastic applause from a crowded ballroom while Mr. Obama received rousing ovations and cheers at beginning and end.

Mr. McCain focused much of his speech on the economy, touting his tax and energy plans as beneficial to small businesses and low- and middle-income Americans. He concluded with a paean to Hispanic-American veterans, recalling a friend and fellow prisoner of war in Vietnam, Everett Alvarez Jr. “When you take the solemn stroll along that wall of black granite on the National Mall,” he said, “it is hard not to notice the many names such as Rodriguez, Hernandez, and Lopez that so sadly adorn it.”

He added: “Those men and women are my brothers and sisters, my fellow Americans, an association that means more to me than any other.”

Mr. Obama also spoke poignantly about the immigrant tradition, recalling Hispanic-Americans he had met on the campaign trail and criticizing the hostility toward immigration that erupted during the campaign last year. “America has nothing to fear from our newcomers,” he said. “They have come here for the same reason that families have always come here, for the same reason that my own father came here from Kenya so many years ago — in the hope that here, in America, you can make it if you try.”

Before Mr. Obama spoke late in the afternoon, a McCain surrogate, Rep. Lincoln Diaz-Balart of Florida, offered a stinging rebuke of the Democrat’s attempt to tout his involvement in the push for immigration reform in Congress.

Mr. Diaz-Balart said Mr. Obama was “absolutely AWOL” on the issue, which Mr. McCain spearheaded along with Senator Kennedy of Massachusetts.

“It’s absurd for him now to try to take credit on that issue when he was nowhere to be seen,” Mr. Diaz-Balart told reporters on a conference call. “He was an absolute non-player.”

“Senator Obama has a steep hill to climb with regard to the Hispanic community,” Mr. Diaz-Balart said, adding that Mr. McCain would benefit from the relative lack of familiarity of Latinos with Mr. Obama, a first-term senator.