Mitt Romney and newly minted running mate Paul Ryan will try to energize supporters in North Carolina Sunday after they hit the road on a bus tour across must-win US states, selling themselves as the duo who can 'save the American dream'.

Fresh from a surprise early morning rollout of Romney's Vice Presidential pick in Norfolk, Virginia, the Republican pair struck out across the state pushing a policy of fiscal responsibility and savaging President Barack Obama as a job-killer bent on changing the country for the worse.

Romney's choice, cast as a bold move by his party, is sure to transform the Presidential race less than three months before November's election, and the two men electrified crowds as they took to the stump.

The campaign also aims to sharply shift debate away from Romney's business record, taxes and image as an out-of-touch multi-millionaire investor and toward larger wholesale issues such as how to revitalize the nation's sputtering economy.

"We can turn this thing around," Ryan, 42, told cheering supporters in Norfolk.

"High unemployment, declining incomes and crushing debt is not a new normal. It's the result of misguided policies," the seven-term congressman from Wisconsin said, in pointed criticism of Obama.

At subsequent Virginia stops, in Ashland and then Manassas, where the campaign said the rowdy crowd topped 8,000, the tip of Ryan's verbal spear grew sharper.

Obama has been pushing a "government-centered society with a government-run economy. It's not working. It's never worked before," Ryan said, rallying supporters in Manassas, outside the capital Washington.

"We were promised equal opportunity, not equal outcomes," he added to huge applause.

In recent weeks, Romney has slumped behind Obama in opinion polls, with the incumbent taking a clear lead nationally and in most of the dozen swing states that will decide the November 6 election.

A Fox News national poll out Thursday put Obama at 49 per cent to Romney's 40, while a CNN poll had Obama at 52 per cent, seven points up on the former Massachusetts governor.

But by picking Ryan, a favorite of small government conservatives, and embarking on a four-day bus tour across battleground states - expanded to five to allow voters in Wisconsin to laud their native son on Sunday - Romney hopes to gain the upper hand in the race.

The pair went on the offensive in Ashland, with Romney ominously criticizing Obama as "a president who's trying to change America ... into something we might not recognize."

He also blamed the Obama campaign for the bitterly negative tone of the race.

By contrast, he praised his running mate as an "intellectual leader in our party," someone who earned respect from Republicans and Democrats alike for having "made friends on both sides of the aisle."

Romney sought to forge Ryan's identity as a responsible small government fiscal hero before the opposition had a chance to do otherwise.

Ryan chairs the House Budget Committee, and earlier this year unveiled a budget plan - widely backed by Republicans - that slashes federal spending, lowers taxes for individuals and corporations and overhauls entitlement programs like the Medicare and Medicaid health care plans.

Obama's Democrats immediately went on the attack, alleging that the cuts would hurt those who rely on such aid to pay for health care, including the elderly.

"Mitt Romney has chosen a leader of the House Republicans who shares his commitment to the flawed theory that new budget-busting tax cuts for the wealthy, while placing greater burdens on the middle class and seniors, will somehow deliver a stronger economy," Obama campaign manager Jim Messina said.

With Congress saddled by its lowest approval ratings on record and Ryan's status as a Washington insider - he was first elected at just 28 years old - he may, like Romney, have his work cut out to appeal to ordinary voters.

But an 'excited' Ryan vowed to 'win this campaign'.

"We've got the wind behind us," he told reporters aboard the campaign plane as it flew from the Washington suburbs to Charlotte, North Carolina.

The decision to pick Ryan was welcomed by Senator John McCain, who lost to Obama in 2008. He said the Republican ticket now featured the "strongest team to return America to prosperity and to defend our interests abroad."

Top conservatives like former secretary of state Condoleezza Rice and potential VP pick Marco Rubio, the freshman senator from Florida, both hailed Romney's choice.

House Majority Leader Eric Cantor said: "Quite simply, Mitt Romney could not have made a finer choice for the future direction of our country."

For his part, New Jersey Governor Chris Christie said the Republican candidates understood "the urgency" with which the country needed to change course.

"The Romney-Ryan team is uniquely positioned to make the tough choices necessary to confront our fiscal challenges and get results," he said.