By Thomas Ferraro and Edith Honan

WASHINGTON (Reuters) - President Barack Obama's Democrats lost a liberal voice and vote with the death of U.S. Senator Frank Lautenberg of New Jersey on Monday, but it was unlikely to shift the balance of power in the Senate on any major issues including immigration reform.

That is primarily because with 45 seats in the 100-member Senate, Republicans already had more than enough clout to block much of Obama's second-term agenda.

New Jersey Governor Chris Christie is expected to name a fellow Republican to temporarily replace Lautenberg until New Jersey voters can elect a new senator as early as this November.

Lautenberg's death at age 89 of complications from viral pneumonia reduced Democrats' control of the Senate to 54-45.

Democrats need 60 votes to clear Republican procedural roadblocks on legislation and nominations, including Obama's picks to the federal courts and his own Cabinet.

"You never want to lose a vote, but I don't think it's going to make much of a difference," a senior Democratic aide said. "We are still going to win the votes we were going to win, and lose the votes we were going to lose."

Because of conflicting state laws, it was not immediately clear if an election to replace Lautenberg would be held this November or in November 2014. Earlier this year he announced he would not seek election to a sixth term in 2014.

Depending on how the dispute is resolved, possibly in court, Christie's pick may serve in the Senate for as few as five months or as many as 17 months.

Lautenberg died as the Senate prepared to complete action this week on a farm bill and begin consideration next week of Obama's top legislative priority, overhaul of the U.S. immigration system.

Backers of both measures predict they will pass with more than the 60 votes necessary. Without Lautenberg's vote, however, some amendments may narrowly pass while others narrowly lose.

Sixty has been difficult to reach on a number of Obama's federal appeals court nominees. It will be that much tougher now.

FOCUS ON IMMIGRATION

Christie, a potential 2016 White House contender, gave no immediate indication of who he might pick.

There was speculation that he may select someone willing to cooperate with Obama, given that the two worked amicably together during the 2012 White House race when Obama helped the governor get federal aid after Superstorm Sandy.

Supporters of the bipartisan immigration bill said they were confident Lautenberg would have voted for the measure.

They also said that even if Christie picked a Republican, the new senator would likely support the bill because of the state's large Hispanic population.

But Matthew Hale, an associate political science professor at Seton Hall University, said it was unclear in what direction Christie would go and how it might affect the immigration bill.

"Christie might even consider someone who is conservative on immigration as a way to appeal to hard-line conservative nationally that don't really like him," Hale said.

New Jersey Lieutenant Governor Kim Guadagno is seen as a possible front-runner for the Senate appointment. New Jersey State Senator Tom Kean and U.S. Representative Chris Smith, both Republicans, are also considered possibilities.

Newark Mayor Cory Booker, a rising black star in the Democratic Party, has long been viewed as Lautenberg's likely elected successor.

Brigid Harrison, a political science professor at Montclair State University, speculated that Christie might cross the political aisle and appoint Booker to the Senate.

"The appointment of Cory Booker could ingratiate him and endear him to a constituency who may not be in the governor's corner - especially African American voters," Harrison said.

"Looking ahead to 2016, this would also give credence to the claim of bipartisanship in New Jersey," Harrison added.

But David Redlawsk, a political science professor at Rutgers University, said picking Booker could hurt Christie.

"If I were Chris Christie, and I was thinking about running on the Republican presidential ticket and facing Republican primary voters, that would just be a political bombshell and the right wing will run against him on it," he said.

Christie, who clashed with Lautenberg over the years, said he would miss him - despite their differences.

"We very often didn't agree, and we had some pretty good fights between us over time - battles on philosophy and the role of government," Christie said, scrapping a prepared speech in favor of a salute to Lautenberg at a women's conference.

"Whenever we lose someone who's committed to public service and has been an honest and dedicated public servant as Senator Lautenberg was it's a loss for everyone," Christie said.

(Additional reporting by Edith Honan and Barbara Goldberg; Editing by Paul Simao, Cynthia Osterman and Xavier Briand)