The United States (US) president is chosen from state-by-state contests, rather than a nationwide popular vote. So battleground states - which are neither reliably Republican nor Democratic - are very important in tight elections, as this year's presidential vote is expected to be.
Reflecting campaign strategy, more than one-fifth of the president's advertising spending has been in Ohio, a state that looms as a must-win for Mr Romney more so than for Mr Obama. Florida ranks second and Virginia third, according to organisations that track media spending and other sources.
About three-quarters of the president's advertising has been critical of Mr Romney as Mr Obama struggles to turn the election into a choice between himself and his rival, rather than a referendum on his own handling of the weak economy.