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WASHINGTON -- President Barack Obama's presidential campaign staff on Wednesday accused former Massachusetts Gov. Mitt Romney of changing his positions on a series of issues, offering a preview of what could be their central argument against the Republican presidential candidate next year.

Obama's campaign said in a conference call with reporters that Romney had shifted his stances on taxes, health care and trade with China, raising questions about what he would do if elected president.

"If you are willing to change positions on fundamental issues of principle, how can we know what you will do as president?" said Obama senior strategist David Axelrod.

The stepped up criticism by Obama's team signaled that they increasingly view Romney as in command of the Republican race, and the flip-flopping charges could serve as a key argument against the Republican's campaign next year.

Romney has led the Republican field in recent polling, delivered strong debate performances and picked up the endorsement of New Jersey Gov. Chris Christie, building a sense of inevitability for his campaign.

Romney's campaign did not immediately respond to Axelrod's comments. The former governor has emphasized his experience in the private sector and questioned Obama's leadership. "He said he'd bring us hope and change. Instead, he's divided the nation and tried to blame other people," Romney said in Tuesday's debate.

Axelrod scrutinized Romney's position on a payroll tax cut for employees, a key part of Obama's jobs plan. Romney said in the Republican debate Tuesday that he supports permanent changes to the tax code and referred to payroll tax cuts as "temporary little Band-Aids."

Democrats circulated a transcript of an August interview in which Romney, when asked about extending the payroll tax cut, said he was "in favor of keeping taxes down and keeping burdens down on American businesses and employers."

Obama and Democrats have called for the extension of the tax cut, which is set to expire at the end of the year. Typical workers would get an extra $1,500 in their paychecks next year under the cut.