* President casts challenger in contradictory terms
* Obama signals he'll make political use of bin Laden death
* For Romney, a softening in tone.
By Steve Holland
WASHINGTON, April 28 (Reuters) - After months of casting Republican Mitt Romney as someone who often changes positions for political convenience, President Barack Obama's campaign is calling Romney a far-right conservative - a contradictory set of messages that essentially invites voters to decide what they don't like about Romney.
And Romney, who has built his campaign around declaring Obama a failure - particularly on the economy - began sounding a more positive note this week, offering hints about his vision for governing if he defeats the Democratic president in the Nov. 6 election.
The subtle changes in tactics by both candidates in recent days are benchmarks for the fall campaign. They symbolize the multiple angles of attack each man plans to use to try to define his rival and appeal to the 20 percent or so of U.S. voters who describe themselves as independent - and who will decide what both sides agree is likely to be a very close election.
And as a new wave of biting video ads from each side made clear this week, this will be a campaign in hyper drive. The election is more than six months away, but with many polls showing Obama and Romney in a virtual tie, voters already are seeing the type of sharp attack ads that typically dominate the final weeks of a presidential campaign.
Just before the first anniversary of the Obama-ordered raid in Pakistan that killed al Qaeda leader Osama bin Laden, the president's campaign released a provocative ad that signaled it would not be shy about making political use of bin Laden's death - or questioning whether Romney would have made the same call.
The Web video, narrated by former president Bill Clinton, takes a direct swipe at Romney by using four-year-old quotes in which Romney questioned whether chasing bin Laden in Pakistan was worth the time and expense.
"The commander-in-chief gets one chance to make the right decision," the ad says. "Which path would Mitt Romney have taken?"
Meanwhile American Crossroads, a Republican group that supports Romney, released a video ad that took aim at Obama's "cool" image, casting the president as a jet-setting celebrity at a time when many Americans are struggling under his economic policies.
"After four years of a celebrity president," the ad asks, "is your life any better?"
After finally knocking out a relatively weak field of conservative challengers, Romney has run into the full force of Obama's campaign.
During the primary season Democrats targeted Romney repeatedly, mostly ignoring his Republican opponents, and tried to label him as a wealthy former private equity executive with a history of being a flip-flopping moderate as the governor of Massachusetts.
But after a campaign in which Romney sought to appeal to conservative Republicans by espousing strict views on limiting immigration, opposing abortion and opposing most government efforts to ease student debt, Obama's campaign is casting Romney as a candidate who has embraced right-wing, extremist views.
Obama told Rolling Stone magazine that he did not believe Romney would be able to disavow the conservative positions he took during the primaries.
"I don't think that their nominee is going to be able to suddenly say, 'Everything I've said for the last six months, I didn't mean,'" the president said.
Democratic strategist Steve Elmendorf rejected the notion that Team Obama should choose between labeling Romney as either a hard-core conservative or a finger-in-the-wind politician.
"They're going to attack him on multiple fronts," Elmendorf said. "This will not be a positive campaign."
But Republican strategist Dave Carney, who advised Texas Governor Rick Perry's 2012 short-lived presidential campaign, sees a Democratic campaign that is "flailing around."
"You can't be a flip-flopper one minute and a hard-core conservative the next," he said.
Romney aides say Obama's tactics are aimed at diverting attention from annual $1 trillion government deficits and an unemployment rate that remains above 8 percent.
"The Obama campaign is like one of those gyrating, intermittent lawn sprinklers, spewing out attacks in seemingly random directions, hoping to get somebody wet," said Romney campaign manager Matt Rhoades.
The challenger continues to load his speeches with attacks on Obama's handling of the U.S. economy and say that the president wants government to have an unacceptably large role in Americans' daily lives.
But now Romney is softening his tone so he doesn't come across so negatively, emphasizing what his priorities would be as president.
During his victory speech after the Pennsylvania primary on Tuesday, Romney made a point of saying he would try to bring Americans together in a way Obama has not.
"Today the hill before us is a little steep but we have always been a nation of big steppers," he said. "Many Americans have given up on this president but they haven't ever thought about giving up. Not on themselves. Not on each other. And not on America."
Romney still has much explaining to do before his vision for governing becomes clear. He has vowed deep spending cuts in the federal budget, for example, but has not outlined which programs he would cut.
THE BUSH FACTOR
During a week in which Obama campaigned before cheering crowds of university students and stared down Republicans in Congress over keeping low rates for student loans, Romney showed some flexibility on student debt, agreeing with Obama's push to extend low rates on student loans.
Republicans acknowledge that to defeat Obama, Romney will need to do more than attack the president on the economy and stress his own record as a corporate executive.
Instead, they say he needs to outline an economic narrative that separates him not just from Obama, but also from the policies of Obama's predecessor, Republican George W. Bush. Obama's campaign has cast Romney's policies as a return to Bush's failed agenda.
This week Romney got advice from a lead editorial in The Wall Street Journal, whose opinion pages typically reflect the thinking of Republican leaders. Among other things, the Journal urged Romney to separate himself from Bush's economic policies to try to inoculate himself from Obama's Bush-Romney linkage.
"Mr. Romney will have to make a case not merely against Mr. Obama's failings," the Journal editorial said, "but also for why he has the better plan to restore prosperity." For more stories on the campaign, click on
(Additional reporting by Sam Youngman; Editing by David Lindsey and Xavier Briand)