Backed by the largest Spanish-language broadcast network in the U.S., a massive campaign by Latino media and grass-roots groups to spur millions of eligible Hispanic residents to become U.S. citizens is showing results that could influence the agenda and outcome of the 2008 election. More than eight million green-card holders — that is, legal permanent residents — are eligible to become U.S. citizens, and the majority are immigrants of Latin American origin, according to U.S. government data. Now, Univision Communications Inc. is using its considerable clout with the Spanish-speaking community in the U.S. to turn this latent voting bloc into...