His speech within sight of the Mexican border was heavy with political overtones for 2012 and beyond, as Mr. Obama sought to reassure increasingly frustrated Latino voters of his commitment to liberalizing immigration laws as a moral and economic imperative, and to blame ?border security first? Republicans for his inability to deliver on that promise.

?We?ve answered those concerns? of Republicans, Mr. Obama said, citing the increase in the past two years in the number of agents, National Guard troops, border fencing, aerial surveillance and deportations of illegal immigrants. ?We have gone above and beyond,? he added, but Republicans keep moving the goalposts ? to the point, Mr. Obama joked, where they will next seek a moat and alligators.

But Mr. Obama?s first visit to the Mexican border as president underscored a tension over his immigration record that colors his re-election prospects: His boasts of strengthening border security wins him no credit among Republicans and only alienates many Latino voters so long as he cannot deliver on his campaign promise to them ? a path to citizenship for the estimated 11 million of immigrants already here illegally.

Mr. Obama?s first stop was an inspection complex, one of the busiest of the 327 official ports of entry to the United States, for cargo, vehicles and even walkers entering from Ciudad Juarez, a sprawling city afflicted by Mexico?s drug wars. Then he spoke in a small national park of America?s heritage as a national of laws and immigrants, against a backdrop of American flags and ? seen in the distance on a sunny day -- a billowing 162-foot-by-93-foot Mexican flag across the Rio Grande.

Mr. Obama did not push such legislation in his first two years, when Democrats controlled Congress, and it has virtually no chance of passage now that Republicans have a House majority. That leaves pro-immigration groups pushing for Mr. Obama to take executive actions at least.

The president?s trip was his fifth event in four weeks to promote his support for the legislation; he previously hosted three meetings at the White House with a wide range of groups and gave the commencement address at Miami-Dade Community College in Florida, where his lines on immigration were among the most applauded.

By the stepped-up activities, including separate efforts by Cabinet members, Mr. Obama?s advisers seek to demonstrate that the president is not just checking a political box by an occasional speech or rally, but is actively pressing the issue ? and making sure everyone knows that Republicans are the ones standing in the way.

Yet Mr. Obama does not plan to introduce legislation. In his speech, however, he reiterated his legislative aims: A path to citizenship for illegal immigrants that would require them to come forward, pay taxes and a penalty, and learn English; legal status to encourage foreign college graduates and other skilled non-citizen residents to remain and start businesses; and the so-called Dream Act, providing citizenship to young people who were brought to the United States as children and receive an education or want to enter the military.

?If they think that this is going to be the thing that mobilizes an increasingly disappointed Latino electorate, I think they?re wrong,? said Frank Sharry, executive director of America?s Voice Voter Education Fund, a nonpartisan group supporting liberalized immigration. ?They are going to have to make some big administrative action to make up for the fact that he promised something big,? he added.

Mr. Sharry?s remarks in an interview before Mr. Obama?s speech here were more barbed than a congratulatory statement he issued after a White House meeting April 19, where he and others urged Mr. Obama to take executive actions. Those would include ending deportation of those who would be covered by the Dream legislation, limiting deportations mostly to those with criminal records, doing more to prosecute employers who seek and exploit illegal immigrants and challenging some anti-immigrant state laws in federal courts.

In 2008, Mr. Obama got 67 percent of the Latino vote against John McCain and needs at least that much in 2012. Latinos are a significant and fast-growing segment of the electorate in critical southern and western swing states like Florida, Arizona, Nevada, Colorado and nearby New Mexico, which received news of the president?s visit here. And while Republicans have dominated Texas since the 1980s, Democrats? hopes of regaining power one day ? likely after 2012 -- rest largely on its expanding Latino population.

Mr. McCain, as a senator from the border state of Arizona, had long supported the sort of legislation Mr. Obama wants ? and former President George W. Bush, a Texan, unsuccessfully sought. But he changed course to win nomination in a Republican Party that has a vocal anti-immigrant constituency. Mr. McCain maintains his ?border security first? stance in the Senate, where he has been a frequent critic of Mr. Obama?s border enforcement record.