WASHINGTON, June 17 (Reuters) - President Barack Obama's  surprise decision to halt U.S. deportations of young undocumented  immigrants has all but killed a Republican effort to fashion  legislation that could have won  political points with Hispanic  voters in November's elections.

Republican Senators, including Marco Rubio, had been working  behind the scenes for months on a bill that would have allowed  some children of undocumented immigrants a chance to stay in the  United States legally while pursuing college or military  careers.

But Obama's announcement has effectively made the Rubio plan  moot, further complicating Republican presidential candidate  Mitt Romney's search for an immigration policy.

"We're re-evaluating our next steps" for the legislation,  said Alex Conant, a Rubio spokesman.

"I think the new reality is that it's unlikely to pass this  year because the politics on both sides have gotten a lot  tougher and the urgency to pass something this summer ... has  now been removed" by Obama's action, he said.

The bill by Rubio and senators Jon Kyl and Kay Bailey  Hutchison would likely have formed the main thrust of Romney's  immigration plan as he tries to close a huge gap in the polls  with Obama among Hispanics.

Hispanic voters' support for Romney is dismal, with Obama  out-drawing him 67 percent to 21 percent, according to a Pew  Research Center poll conducted last month. Hispanics are a  fast-growing minority, now 51 million strong in a country of 309  million people, and most U.S. undocumented immigrants are Hispanic.

But even before Obama's order to stop deportations of about  800,000 young undocumented immigrants, the Republican senators were  struggling to overcome opposition to their bill from  conservatives in Congress. Rubio was unable to commit to  introducing legislation this year.