WASHINGTON (AP) -- Fresh off a decisive victory in Illinois, Mitt Romney on Wednesday won critical establishment support from former Florida Gov. Jeb Bush as he looks to unite the Republican Party behind his candidacy. Romney said he's "almost there" after pursuing the GOP nomination for six years, and there are fresh signs that big GOP donors and other party figures will follow Bush's lead after sitting on the sidelines for much of the primary season.

The son of one president and the brother of another, Bush had stayed out of the race for months. Some party elders publicly had urged him to jump into the race when it looked like Romney was having trouble closing the deal. On Wednesday, a day after Romney won Illinois by 12 points, Bush signaled that was no longer the case.

"Now is the time for Republicans to unite behind Governor Romney and take our message of fiscal conservatism and job creation to all voters this fall," Bush said in a written statement that suggested the race is all but over. He congratulated the other Republican candidates "for a hard-fought, thoughtful debate and primary season."

Romney had emailed his supporters Tuesday night that his Illinois win "means we are that much closer to securing the nomination, uniting our party, and taking on President Obama." He urged the party to fall in line behind his bid, saying "We are almost there."

The former Massachusetts governor and his allies spent hundreds of thousands of dollars more than Santorum and his backers in Illinois, and it showed in the results: Romney was beating Rick Santorum by 47 percent to 35 percent.

Campaign finance reports released Tuesday showed that big donors to a GOP political organization founded by political strategist Karl Rove have boosted their financial support for Romney in recent weeks.

For all that money, though, Romney's Illinois win was a victory without an electrified electorate: Turnout seemed likely to be among the lowest in decades: Officials in several election districts said turnout hovered around 20 percent.

"You could draw a bigger crowd at a Green Bay Packers rally in downtown Chicago than what Mr. Romney delivered to the polls," Democratic Sen. Dick Durbin of Illinois said on CBS' "This Morning."

Romney was the clear favorite among Illinois Republicans who were most concerned about picking someone who is capable of taking on President Barack Obama in the fall. Romney's wife, Ann, suggested earlier this week that it was time for the party to coalesce behind him. And in an appeal to the centrist independents who will decide the general election, Romney pledged Tuesday to work with Democrats or "die trying."

"Tonight was a primary, but November is a general election. And we're going to face a defining decision as a people," Romney said during a victory speech to supporters. "We know what Barack Obama's vision is. We've been living it these last three years. My vision is very, very different."

Romney picked up at least 41 delegates in Illinois, according to initial results, adding to his delegate lead and making it that much harder for any of his rivals to deny him an opportunity to take on the president in November.

Obama, for his part, headed West on Wednesday to Nevada, New Mexico and Oklahoma City on a trip aimed at answering critics of his energy policies, sure to be a key issue in the fall campaign. His first stop was a plant in Nevada that uses solar panels to power homes, part of an effort to highlight the president's programs to expand renewable energy sources.

The president's GOP critics poked back at him before Obama was even on the plane out of Washington: Newt Gingrich issued a statement saying Obama was answering a real-world problem with a "solution that is totally disconnected from the practical realities of the world and has little chance of success." Crossroads GPS, the nonprofit arm of a Republican super PAC, launched an ad on TV stations in the areas that Obama was to visit and on national cable channels faulting the president for "bad energy policies" that are driving up gasoline prices.

Romney was moving on to Maryland, but opened Wednesday by tweeting a "Happy Anniversary" message to his wife, Ann, complete with a wedding photo from 1969. His campaign released a web video in which Ann Romney recounts the details of their dating-to-marriage story.

Polls show Romney has the advantage heading toward Maryland's April 3 primary. But the South, where Louisiana votes on Saturday, has proven less hospitable to Romney.

Santorum, who hopes to rebound in Louisiana, sounded like anything but a defeated contender Tuesday night as he spoke to supporters in Gettysburg, Pa. He said he had outpolled Romney in downstate Illinois and the areas "that conservatives and Republicans populate."

"We're very happy about that and we're happy about the delegates we're going to get, too," he said before invoking Illinois-born Republican icon Ronald Reagan, the actor turned president. "Saddle up, like Reagan did in the cowboy movies."

Gingrich didn't speak to supporters Tuesday, instead issuing a written statement. Texas Rep. Ron Paul has yet to win a state.

Romney triumphed in Illinois after benefiting from a crushing 7-1 advantage in the television advertising wars, and as his chief rival struggled to overcome self-imposed political wounds in the marathon race to pick an opponent to Obama.

Most recently, Santorum backpedaled after saying Monday that the economy wasn't the main issue of the campaign. "Occasionally you say some things where you wish you had a do-over," he said later.

Romney has 563 delegates in the overall count maintained by The Associated Press, out of 1,144 needed to win the nomination. Santorum has 263 delegates, Gingrich 135 and Paul 50.

After the Louisiana primary, a 10-day break follows before Washington, D.C., Maryland and Wisconsin hold primaries on April 3.

Santorum is not on the ballot in the nation's capital. Private polling shows Romney with the edge in Maryland, and the pro-Romney super PAC Restore Our Future launched a television ad campaign in the state during the day at a cost of more than $450,000.

Wisconsin shapes up as the next big test between Romney and Santorum. Republican politics there have been roiled recently by a controversy involving a recall battle against the governor and some GOP state senators who supported legislation that was bitterly opposed by labor unions.

Already, Restore Our Future has put down more than $2 million in television advertising across Wisconsin. Santorum has spent about $50,000 to answer.

Neither Gingrich nor Paul campaigned extensively in Illinois. Gingrich has faded to near-irrelevance in the race, but remains defiant.

"To defeat Barack Obama, Republicans can't nominate a candidate who relies on outspending his opponents 7-1," he said in a statement Tuesday night. "Instead, we need a nominee who offers powerful solutions that hold the president accountable for his failures."

Gingrich said his campaign will spend the time leading to the party convention "relentlessly taking the fight to President Obama."

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