Hispanic voters are the fastest-growing voting bloc in the U.S. A record 12.2 million Latinos are expected to cast ballots this year.
More important, Latinos will cast a lot of those votes in swing states.
Hispanic votes helped Mr. Obama turn New Mexico, Colorado, Florida and Nevada from red to blue in 2008 and may have also made a difference in Virginia and North Carolina, two other Republican strongholds the president won.
In all, Mr. Obama received 67% of the Hispanic vote. Mr. Romney told donors in Florida if he can't chip away at that support, "it spells doom for us."
Lionel Sosa, owner of a Hispanic advertising firm in Texas and adviser to George W. Bush and John McCain, said Mr. Romney has to distance himself from the hard-line stance on immigration he adopted when he was battling primary rivals.
The Obama campaign spent $25 million in 2008 on Spanish-language ads. Mr. Sosa said Mr. Romney has to pour money into both Spanish-language ads and English-language ads that appeal to Latinos, as most Hispanic voters are English-dominant.
But Mr. Sosa said the best thing Mr. Romney could do is pick a Hispanic running mate, preferably Sen. Marco Rubio, a Tea Party favorite from the swing state of Florida.
"If Romney does all of these things, he can get 50% of the Latino vote," Mr. Sosa said. "But if the election were held today, he would lose."
Mr. Obama also has to work hard to shore up his popularity among Latinos, many of whom are dismayed that the president has set records for deportations and failed to spend political capital on a comprehensive immigration bill.
The president launched "Latinos for Obama" in April and ran the first Spanish-language ad of the 2012 presidential race in Florida.
Meanwhile, the Republican National Committee has rolled out a "Hispanic Outreach Program" that sent organizers to Florida, Nevada, Colorado, New Mexico, Virginia and North Carolina.
David Candelaria, general manager of a Univision station in El Paso, Texas, that has about 20,000 viewers in New Mexico, said both campaigns as well as some Democratic PACs have contacted him about advertising.
"They're talking, but nobody has placed anything yet," Mr. Candelaria said. "We're thinking it's going to happen in late May or early June. And we're expecting a lot more buys than four years ago."