LAS VEGAS, Nevada — Boosted by rising poll numbers, US President Barack Obama courted swing-state voters Thursday, after offering a disdainful critique of his potential Republican general election foes.

Obama, driving home the theme of his State of the Union speech -- a call for a fair economy to restore the "basic promise" of America -- previewed a new energy plan, after an overnight stay in the gamblers paradise of Las Vegas.

A new poll meanwhile suggested that recent signs of improving economic conditions might be at last filtering through to voters, in a way which could ease his tough passage to his hoped-for second White House term.

Obama pressed on with his five-state, three-day swing through key electoral territory as Republicans geared up for a debate Thursday in Florida which could sway their critical next nominating contest in the Sunshine State.

He declined to say which one of the dueling Republican candidates he would prefer to meet in November -- even as Mitt Romney, the one-time favorite for the nomination -- waged a desperate tussle with Newt Gingrich.

"I'll let them determine who their standard bearer is going to be. Until the Republicans have a nominee, we don?t have a campaign," Obama said in an interview with the Spanish-language Univision network.

But in a rare comment on the Republican field, Obama signaled that whoever he met in November would face a fight over American values.

"Whether it's Mr. Romney or Mr. Gingrich or Mr. (Rick) Santorum or whoever else they might decide to select, they represent a fundamentally different vision of America," Obama told Univision.

"It's not the bold generous forward-looking optimistic America that I think built this country."

On a three-day trip that includes Nevada, Arizona, and Colorado, states with large Hispanic populations, Obama pointed out that both Romney and Gingrich had promised to veto the Dream Act, which would help illegal immigrants brought to America as children, towards legal status.

Hispanic voters, for whom immigration reform is a predominant issue, are an increasingly important demographic in US elections, especially in western states, and Obama needs the community to flock to his banner to win reelection.

But despite his promises to push comprehensive immigration reform to move 10 million illegal immigrants out of the shadows, Obama has been unable to take significant steps, partly due to Republican opposition in Congress.

While Obama kept up the fiction that the election race will not begin until he has a Republican opponent, Romney did not hold back from roasting the president.

"He keeps saying the same things and waking up and the same things are going on, nothing changes," Romney said at a printing works in Jacksonville, Florida which is in the process of shutting down.

"He said he was the candidate of change but you still have 25 million people out of work, you still have almost 10 percent unemployment here in Florida,

"You still have home values down and continuing to go down, you still have a record number of foreclosures in Florida."

Despite Romney's view of Obama's record, the president had some good news Thursday on the electoral issue that could decide his fate -- the economy.

A Wall Street Journal/NBC News poll found that for the first time in seven months, more people approved of Obama's job performance than did not -- 48 percent to 46 percent.

Some 45 percent said they approved of Obama's handling of the economy -- up six percent from a month ago. Sixty percent said the country was on the wrong track -- down from 69 percent in December and 74 percent in October.

The right track/wrong track measure is seen as a key indicator of voter sentiment heading into an election.

Obama appeared at a UPS plant in Las Vegas to promote one of his State of the Union initiatives -- a plan to tap a 100-year supply of natural gas which he said could create 600,000 jobs.

He also proposed incentives to encourage the use of medium- and heavy-duty trucks that run on alternative fuels.

The president also unveiled a plan to open up 38 million acres of the Gulf of Mexico for exploration and development.

Obama has been under fierce fire from Republicans over his energy policies, including investments in failed alternative energy firms, his cancellation of a oil pipeline project from Canada through the US heartland and for not opening wider swathes of onshore and offshore waters to oil exploration.