Presidential candidates of both parties have long vied for Latino voters, but this is a midterm year and a new report from America's Voice, a group working for comprehensive immigration reform, suggests Latino voters will be a potentially decisive force in 37 House and Senate races, plus contests for governor in California, Colorado and Texas.

The report says Latino voters may be pivotal in eight Senate races, in Arizona (where Republican Sen. John McCain is trying to keep his seat), California, Colorado, Connecticut, Florida, Illinois, Nevada and New York. It says the group could also decide 29 House races in those states and in New Mexico, Pennsylvania, Texas and Virginia. That includes seven House races in Florida alone, and four each in Arizona, California and Illinois.

Latinos are the fastest growing part of the population. Their voting participation grew by one-third between 2004 and 2008, Janet Murguia, president of the National Council of La Raza, a Hispanic advocacy group, told reporters Monday. President George W. Bush captured 44 percent of their vote in 2004 compared with 53 percent for Democrat John Kerry, exit polls showed. In 2008, McCain only won 31 percent compared with 67 percent for President Obama .

Frank Sharry, executive director of America's Voice, which promotes comprehensive immigration reform, said Latinos make up at least 25 percent of the population in nearly one in five congressional districts and more are registering to vote all the time. He predicted decisions by Latino voters to turn out or not, and to vote for Republicans or Democrats, will have "huge impact" on these 40 congressional and gubernatorial races.

Sharry, Murguia and Eliseo Medina, international executive vice president of the 2.2-million-member Service Employees International Union, said that while the economy is the top concern of Latino voters, they predicted Democrats will have a hard time firing up Latinos to vote if they don't achieve comprehensive reform of the immigration system this year.

"If it doesn't happen there's no question that it will affect Latino turnout. There will be a deep sense of disappointment and that will be reflected on Election Day," Murguia said. Medina said SEIU would tell its members that "this is not the end of the road" and stress that their votes would affect the future of reform. "We're not going to be shy about pointing out and calling out which party tried to pass immigration reform and which party was an obstruction," he said.

Obama called for continued work on immigration reform in his State of the Union address, but he did not say he wanted to see it passed by any particular date. Sharry said the White House continues to mobilize for a bipartisan push on the issue this year. "The final chapter has not been written," he said. "It's still alive for 2010 and we're not prepared to give up."