President Barack Obama waves as he is introduced at a campaign event in Milwaukee (AP)
President Barack Obama and Republican rival Mitt Romney have attacked one another in frenzied campaigning as the presidential election enters the closing stages.
Polling showed the candidates in a virtual tie for the popular vote nationwide, with Mr Obama holding a slight lead in the all-important battleground states that will decide the outcome.
Beyond the nine so-called swing states, Mr Romney was making a last-minute play for traditionally Democratic Pennsylvania with a massive advertising foray and a campaign appearance with running mate Paul Ryan in the state, his first stop there this autumn.
The Republican ticket cast the late push into Pennsylvania as a sign that Mr Romney had momentum and a chance to pull away states that Obama's campaign assumed it would win handily.
Obama senior adviser David Plouffe told ABC television that Romney's move was "a desperate ploy at the end of a campaign," given the Democrats' million-voter registration advantage in Pennsylvania.
Nevertheless, Mr Obama was responding by dispatching deeply popular former president Bill Clinton to the state for a four-city sweep through Pennsylvania.
In heavily Democratic Cleveland, Ohio, Mr, Romney slammed Mr Obama for what he termed the president's failure to end the partisan gridlock that has brought Washington to a virtual legislative impasse and for breaking his promises to vastly improve the economy while cutting the national debt.
"Talk is cheap," Mr Romney said in the Lake Erie industrial city. "But let's look at the record." He went on to lash the president for what Republicans claim are a series of broken Obama promises on both the economy and changing the political climate in the country.
Regardless, national opinion polls showed the race for the popular vote in Tuesday's election so close that only a statistically insignificant point or two separated the two rivals.
The final national NBC/Wall Street Journal Poll showed Mr Obama getting the support of 48% of likely voters, while Mr Romney receives 47%. The poll had a margin of error of 2.55 percentage points.