John McCain and Barack Obama challenged each other for the support of Hispanic voters Saturday, as they made back-to-back speeches to the same group of influential Latino officials in Washington, D.C.

The Latino constituency could be pivotal in the fall. To court it, both candidates pledged to pursue comprehensive immigration reform, and challenged each other's commitment to it.

McCain spoke first and said immigration reform "will be my top priority yesterday, today and tomorrow." He highlighted the unsuccessful immigration legislation he helped author in the Senate.

"I know this country ... would be the poorer were we deprived of the patriotism, industry and decency of those millions of Americans whose families came here from our hemisphere -- Mexico, Central and South America. I will honor their contributions to America for as long as I live," McCain told the group, the National Association of Latino Elected and Appointed Officials.

McCain pointed to his efforts to reform immigration laws. He acknowledged the economic importance of immigrant laborers while saying those who are here illegally should be dealt with.

McCain, who was interrupted four times by anti-war protesters, also made a patriotic appeal to the Latino group. He recalled how he refused early release when he was a prisoner of war in Vietnam because he would not leave behind those who had been imprisoned longer. One of those fellow prisoners was a Mexican-American.

"To leave him behind would have shamed us," McCain said.

Obama later noted that McCain's focus on immigration reform was not quite as sharp during the GOP primary.

During a January debate in California, McCain repeatedly refused to say whether he'd vote for his own proposal if it came to the Senate floor -- some critics have said his reform measures amounted to amnesty since they would provide a pathway to citizenship for millions of undocumented immigrants.

"It will not be voted on," McCain reasoned, arguing that Americans made clear they want the border secured first.

"(McCain) deserved great credit as a champion of comprehensive reform. I admired him for it," Obama told the same group of Latino officials Saturday. "But what he didn't mention is when he was running for his party's nomination, he walked away from that commitment. ... If we are going to solve the challenges we face, we can't vacillate."

Obama said the country still needs such reform to secure the borders but also punish employers who exploit immigrant labor and provide a pathway to citizenship for the millions of undocumented residents already here.

"That has to be one of our priorities as well," Obama said to loud applause. "And I will say it now and I will say it after I'm president."

Responding to Obama's criticism, McCain's campaign issued a statement that said Obama "worked to kill" last year's Senate legislation by voting for amendments to it that Democratic sponsors opposed.

Obama was making the first of three scheduled speeches to Hispanic organizations in less than a month. He plans to speak to two other major groups in July. He also conducted interviews with Spanish-language news outlets Saturday.

Obama spokesman Robert Gibbs told FOX News on Saturday that Latino voters are engaged with Obama's candidacy, not McCain's.

"Senator McCain tailors his message to whatever group he's speaking to," he said.

"If you look at the recent polling, Barack Obama has a huge lead among Latino voters, because voters understand that what Barack Obama's agenda is -- that is a quality of education, that is health care ... comprehensive and real immigration reform."

A recent AP-Yahoo News poll showed that Obama leads McCain among Hispanics, 47 percent to 22 percent with 26 percent undecided.

Still, Obama doesn't have a lock on this volatile group. During the Democratic primary, Hispanics preferred rival Hillary Clinton to Obama by nearly 2-to-1.

McCain, for his part, senses opportunity and is hoping to build on Republicans' recent inroads in this Democratic-trending group.

President Bush captured about 40 percent of the Hispanic vote in 2004, the most ever for a GOP presidential candidate. His Democratic rival John Kerry won 53 percent, down from the 62 percent former Vice President Al Gore got in 2000.

Clinton, who appeared with Obama at their first joint campaign rally Friday in New Hampshire, warmed up the crowd at NALEO on Thursday in an effort to convince her former backers to line up behind the presumptive Democratic nominee.

"Every issue you care about personally ... is really at risk," Clinton told the Latino group Thursday. "We cannot afford four more years of the same. ... And therefore we have to be determined to chart a new course and we cannot do that without electing Barack Obama our next president."

The Associated Press contributed to this report.