by Miriam Jordan

A coalition of Latino groups backed by the Census Bureau will launch a campaign Thursday to pull off an elusive feat: getting an accurate count of Hispanics in the U.S.

The Hispanic population has exploded since the 2000 census, growing 33% to an estimated 47 million. But it is also one of the hardest groups to track.

Hundreds of thousands of Hispanics are migratory workers. Several million are in the country illegally and reluctant to fill out official forms. Language barriers persist, and the foreclosure crisis has left many Hispanic families without a mailing address.

"This is adding up to a perfect storm when it comes to the census on Latinos," said Arturo Vargas, executive director of the National Association of Latino Elected and Appointed Officials Educational Fund, a nonpartisan group known as Naleo, which is spearheading the campaign.

The outreach is also being led by Spanish-language media, unions and grass-roots groups. Dubbed "Ya Es Hora. Hagase Contar!" (It's Time, Make Yourself Count!), the effort is designed to encourage all Hispanics, even immigrants in the U.S. illegally, to fill out census forms.

An accurate count could bolster the swelling political clout of Hispanics. The decennial count will determine the allocation of billions in federal and state funds each year. Data from the questionnaire will also be used to determine reapportionment for congressional and state legislative districts.

The census campaign seeks to counter calls for a boycott of the tally by groups angry at President Barack Obama's inaction on legislation that could legalize about 12 million undocumented immigrants.

The coalition of Latino groups is coming off a trio of victories. Over the past couple of years, it successfully recruited many Latinos to become U.S. citizens and also orchestrated a major voter-registration drive, followed by a massive voter-turnout effort.

Univision, the largest Spanish-language broadcaster in the U.S., will bombard its three television networks and its radio airwaves with public-service announcements on the census. Its reporters will weave motivational messages into news shows.

"People will be hearing about the census on their way to work, at church and at union meetings," said Eliseo Medina, executive vice president of the Service Employees International Union, which represents janitorial, health-care and food-service workers. "We are going to get the word out until people are sick and tired of hearing about the census," Mr. Medina said.

The Census Bureau is coordinating with the Latino groups and separately is investing $312 million on advertising in 28 languages.

Much of the advertising will be directed at so-called "hard to count" communities, census tracts with a history of low participation that typically include immigrant enclaves. For the first time, the Census Bureau is mailing bilingual questionnaires to about 13.5 million households in areas where at least a fifth of them use Spanish as their primary language.

In the Los Angeles area alone, the Census Bureau has 300 staff dedicated to working with churches and community groups to get at hard-to-reach immigrants.

"We want to get down as low as we can go and get into these communities," said James Christy, director for the Los Angeles region.

At a Brazilian festival in Los Angeles earlier this month, Census Bureau staffer Jose Mayorga told visitors, "We don't ask about immigration status."

He distributed fact sheets in Portuguese that stated in capital letters: "CONFIDENTIALITY IS THE LAW!!!!"

One group that has called on Hispanics to boycott the census is the National Coalition of Latino Clergy and Christian Leaders. Its leader, the Rev. Miguel Rivera, claims that data from the 2000 census indicating a surge in the Hispanic population spurred anti-immigrant politicians, particularly in states like Georgia, to crack down on illegal immigrants.

"We don't want to help such states gain seats in the House," Mr. Rivera said.

Write to Miriam Jordan at miriam.jordan@wsj.com