Republican presidential candidate Mitt Romney (left) stands with Congressman Paul Ryan after introducing him as his vice-presidential running mate during a campaign event at the retired battleship USS Wisconsin in Norfolk, Virginia.

Mooresville, North Carolina - Republican Mitt Romney's presidential campaign took on a buoyant tone on Sunday, as Romney and his new vice-presidential running mate, Paul Ryan, basked in the support of cheering crowds in North Carolina.

The rallies, starting with one at a NASCAR training centre, showed how Romney's selection of the Wisconsin congressman as his running mate has injected new energy into a campaign that had struggled to move beyond Democrats' efforts to cast Romney as a wealthy former private equity executive who cannot relate to middle-class Americans.

“Are we going to win North Carolina?” Ryan told a boisterous crowd in Mooresville.

“Yeah!” the crowd shouted back.

“We can either stay on the current path we are on,” Ryan said. “A nation in debt, a nation in despair, a nation of unemployment... Or we can change this thing and get this country back on the right track.”

Hours earlier in Virginia, where Romney had introduced him as the No. 2 on the Republican ticket on Saturday, Ryan, 42, the chairperson of the US House of Representatives Budget Committee, told reporters that being thrust into the presidential campaign was “very exciting”.

We're going to win this campaign. We've got the wind behind us,” Ryan said.

Romney, 65, seemed relieved to have a sidekick to end what he has called the “two against one” dynamic of the race, with President Barack Obama and Vice-President Joe Biden on one side and Romney on the other.

“It's a far more compelling dynamic than just being out there on my own,” Romney said late on Saturday.

But it also was evident that Romney's selection of Ryan - known for his sweeping budget plan to reduce government spending and debt by trimming taxes and revamping Medicare and other social programmes - is going to raise a series of hurdles for Romney's campaign as it sprints toward the November 6 election against Obama.

In choosing Ryan, Romney is attaching himself to Ryan's controversial budget plan, which has long been blasted by Democrats who say it would dismantle popular social programmes that help the elderly and the poor.

Ryan's selection also suggested that Romney is tackling a prickly task during an intense, nasty and likely close race for the White House: asking Americans to consider tough questions about the future of Medicare, the nation's government-backed health insurance programme for the elderly, and a range of other programmes plagued by runaway spending.

In previous elections, a candidate's desire for such a not-so-popular conversation with Americans has not gone well.

In 1984, Democrat Walter Mondale emphasised the need for higher taxes and was swamped at the ballot box as voters re-elected Republican Ronald Reagan.

On Sunday, Republican National Committee Chairperson Reince Priebus cast Romney's campaign as the one being honest with Americans about the nation's fiscal future and said Obama's team is more interested in attacking Romney.

Selecting Ryan shows that Romney “has the leadership and courage to present to the American people a real contrast and a real debate that the American people deserve”, Priebus said on NBC's Meet The Press.

Even so, Romney's campaign team continued to stress that Romney would propose his own fiscal plan, suggesting that it did not want the former Massachusetts governor to be tied to everything in Ryan's budget plan.

“The thing you have to remember about these campaigns is that Governor Romney is at the top of the ticket, and that Governor Romney's vision for the country is something that congressman Ryan supports,” Romney spokesperson Kevin Madden said.

Democrats' efforts to cast Ryan - and, by extension, Romney - as a threat to Medicare could be key in the election.

Ryan's plan calls for an end to the guaranteed benefit in Medicare and replaces it with a system that would give vouchers to recipients to pay for health insurance.

The risk in such a plan is that if healthcare costs rise faster than the value of the vouchers, seniors would have to pay the difference.

Obama senior campaign adviser David Axelrod said on Sunday on Meet The Press that the Medicare changes supported by Ryan would put the healthcare programme, which polls indicate most Americans do not want changed, into a “death spiral”. - Reuters