Republican U.S. presidential candidate Mitt Romney and his new running mate are casting themselves as the duo to bring back America's economy.

Romney and Representative Paul Ryan campaigned Sunday in North Carolina and then in Ryan's home state of Wisconsin. Both states are considered key battlegrounds in the race for the White House.

Romney is hoping Ryan's work in Congress to cut spending and create smaller government will help energize the Republican Party's conservative base.

They launched their four-day, multi-state campaign bus tour on Saturday, immediately after Romney announced the 42-year-old congressman as his vice presidential pick.

President Barack Obama's campaign blasted the choice, saying Ryan proposed a budget that would cut taxes for the wealthy and hurt the middle class and seniors.

Romney, the presumptive Republican nominee, will face President Obama, a Democrat, in the November 6 election.