DALLAS: American-born Manuel Rendon came of legal age in the midst of rallies around the country for immigration reform and quickly registered to vote.

"Once I turned 18, I knew that was the one way to have my voice heard and to really make an impact. So it wasn't just my right, it was my duty," said Rendon, the son of Mexican immigrants.

For years, Hispanics have lagged behind other voters in their political clout, in part because so many of them were under 18.

But now, 400,000 U.S.-born Latinos a year are joining the voting-age population by turning 18. More than 5 million Latino citizens, either U.S.-born or naturalized, were ages 18 to 29 as of September 2007, according to the Pew Hispanic Center.

The power of that fast-growing slice of the Latino vote may soon be put to the test in Texas, where Democratic presidential candidates Hillary Rodham Clinton and Barack Obama are competing fiercely for the support of Hispanic voters in the state's March 4 Democratic primary.

About 20 percent, of 2.6 million, registered voters in Texas have Hispanic surnames, and about a third of the state's eligible Hispanic voters are 18 to 29.

"If they turn out in bigger numbers than they have in the past, it could be a real turning point. It's this very large and growing untapped pool," said Roberto Suro, a founder of the Pew Hispanic Center and a journalism professor at the University of Southern California's Annenberg School for Communication.

Three other states hold primaries Tuesday. Ohio has a large cache of delegates, but Hispanics comprise only 1.5 percent of the electorate there; 5.6 percent of eligible voters are Hispanic in Rhode Island, and 0.6 percent in Vermont.

Nationally, the young Latino population is growing so quickly that older community organizations are having a hard time keeping up, said Maria Teresa Petersen, executive director of Voto Latino, a nonpartisan group.