WASHINGTON — President Barack Obama's reelection campaign on Monday charged that top Republican White House hopefuls Mitt Romney and Newt Gingrich had alienated Latino voters with "extreme rhetoric" on immigration.

In a memo sent to journalists, the campaign said Romney and Gingrich "may very well have already sealed the political fate of their party with the Hispanic electorate -- the fastest growing voting bloc in the country."

"Their extreme rhetoric on immigration during the televised debates has rejected our history as a nation of immigrants and alienated millions of Hispanic voters nationally," said "Obama for America" pollster Sergio Bendixen and Director of Hispanic Press Gabriela Demonzain.

The Democratic assault on Romney and Gingrich came one day before Florida's nominating primary, a key battle in the war for the Republican nomination to take on Obama in the November elections.

Roughly 22.5 percent of the population in Florida, a critical battleground, is of Hispanic origin, as is 16.3 percent for the United States as a whole, and Obama enjoys a robust lead over his potential rivals nationwide among the group.

He would beat Romney 67-25 percent and Gingrich 70-22 percent in a hypothetical match-up today among Hispanics, according to a public opinion poll by Univision/Latino Decisions.

The survey, which had an error margin of plus or minus 4.4 percentage points, found a closer fight in Florida, where Obama would beat Romney 50-40 percent and best Gingrich 52-38 percent.

Romney, who is expected to beat Gingrich and all but seal up the nomination in Florida, has come out against a legislative proposal called the "DREAM Act" that would offer a pathway to US citizenship for some undocumented immigrants brought to the United States as children.

"Many Hispanics see Romney's strong opposition to the DREAM Act and to any type of comprehensive immigration reform as more of a demagogic appeal to Tea Party voters than an attempt to formulate a responsible policy," said the memo.

With the sour US economy weighing on his reelection bid, Obama has taken pains to shore up his support with key Democratic constituencies, including Hispanic voters who could make the difference in a handful of critical states.