APPLETON, Wis. - Closing in on the Republican nomination, presidential hopeful Mitt Romney turned his attention on Friday to a general election fight with President Barack Obama over the U.S. economy.

At the start of a five-day blitz in Wisconsin that could prove crucial to the Republican race, Romney ignored his three party rivals and tried to position himself as the presumed nominee to face Obama in the Nov. 6 election.

A new poll put Romney ahead of conservative Rick Santorum in Tuesday's Wisconsin primary, where the front-runner would take a large step toward becoming the nominee if he wins.

Romney, the former head of a private equity firm, said Americans face a stark choice in the November election over which direction to take the country in.

“He (Obama) has spent the last four years laying the foundation for a new government-centered society. I will spend the next four years rebuilding the foundation of our opportunity society, led by free people and free enterprises,” he said.

The prolonged Republican campaign, pitting Romney against a series of conservative challengers who have fallen short, is entering what could be a decisive phase.

A Wisconsin victory on Tuesday, combined with potential wins on that day in Maryland and Washington, D.C., would put more distance between Romney and Santorum and make it nearly impossible to catch him. Newt Gingrich and Ron Paul are also in the race but not campaigning as heavily.

The White House hopefuls are in a state-by-state battle to accumulate the 1,144 delegates needed to win the nomination. Romney leads with an estimated 565 delegates, according to Real Clear Politics, followed by Santorum with 256, Gingrich with 141 and Paul with 66.

The next big primary date, April 24, looms as increasingly important with six states to vote and Romney expected to win at least five of them. Santorum's home state of Pennsylvania is the only question mark and Romney plans to campaign there.

But Tuesday's vote will be significant as well. Romney got a big boost in Wisconsin with the endorsement of the Midwestern state's Representative Paul Ryan, the powerful chairman of the U.S. House of Representatives Budget Committee.

Ryan, who introduced Romney in Appleton, is a popular figure among conservatives and a potential vice presidential choice for the Republican nominee. His backing could further help Romney with those conservatives who fear he is too moderate and untrustworthy.

Entering key phase

Speaking to Fox News, Ryan said the Republican race has gone on long enough and that it is time to rally around Romney.

“I think we're entering a phase where it could become counterproductive if this drags on much longer,” he said.

Romney enters the Wisconsin campaign with a lead in the state's polls over Santorum.

The former Massachusetts governor won support from 40 percent of likely primary voters while former Pennsylvania Senator Santorum got 33 percent in an NBC/Marist poll on Friday.

Texas Representative Paul was at 11 percent and former House of Representatives Speaker Gingrich had 8 percent.

A Romney victory in Wisconsin, said Republican strategist Rich Galen, would be further evidence of what many Republicans think, “that this is essentially over, that Romney has gone from the front-runner to being the prohibitive nominee.”

Romney's speech in Appleton was a sign that his campaign believes it is time to begin focusing even harder on Obama, who will have the advantage of incumbency and many millions of dollars to spend on his re-election effort.

Romney made a case that Obama's economic strategy has been a “bust,” and laid out a number of specific accusations, that 800,000 jobs have been lost under Obama, 46 million people now live in poverty, just as many on food stamps, and 2.8 million homes have been foreclosed upon.

He said Obama has become disconnected with everyday Americans - the same charge Democrats make about the wealthy Romney.

“He (Obama) remains surrounded by true believers who attack anyone who challenges their power. And, as we see each day, they will fight even more fiercely to hold on to that power. All of this is to be expected,” said Romney, reading his speech from a teleprompter.

“That power loves power and never lets go easily is hardly new. And that a White House has lost touch, well, I think we've seen that once or twice before,” he said.

To underscore the Romney case against Santorum, his campaign played over the sound system the Kenny Rogers' song, “The Gambler.” Former President George H.W. Bush had cited a “you have to know when to fold ‘em” line from the song in endorsing Romney on Thursday and urging Romney's rivals to step aside.

Santorum has vowed to stay in the race and is unlikely to withdraw no matter the Wisconsin outcome, but a loss could impact the amount of money he is able to raise and put further pressure on him.

“At some point reality has to set in and he's got to realize that there is no path to victory here,” said Jennifer Duffy, political expert at the non-partisan Cook Political Report. “Either come up with a different rationalization for staying in, which I think will be tough, or get out and call it a day.”