Barack Obama worked to hang on to his slim lead in the crucial battleground state of Ohio and bounce back from a disappointing debate performance that re-energised the campaign of Republican challenger Mitt Romney.
Mr Romney was also in Ohio, intensifying his efforts to seize a Midwestern state that could decide the close race for the White House.
A victory in Ohio is critical for both candidates but especially for Mr Romney. No Republican has ever won the presidency without carrying Ohio.
Mr Obama has held a polling edge in the state for weeks but there are signs that his advantage is narrowing. A new CNN poll showed him leading Mr Romney 51% to 47% among likely Ohio voters. Republican strategists said the race was even closer - within 1% - as the candidate enjoyed a post-debate surge of support.
The President is chosen in state-by-state elections, not a national popular vote. Each state gets one electoral vote for each of its representatives in the House and Senate. There are 538 votes in the Electoral College, and a candidate must have at least 270 to win. Except for Maine and Nebraska, states award all their electoral votes to the candidate who wins the state.
The system makes states such as Ohio - with its 18 electoral votes and a population that is neither reliably Republican nor Democratic - fierce battlegrounds. Ohio decided the 2004 race in favour of Republican George Bush over Democrat John Kerry.
Both candidates campaigned hard in the state on Tuesday, the last day of voter registration ahead of election day, four weeks away.
Mr Obama told young voters at a large rally at Ohio State University in Columbus not to wait or delay their vote, directing them to buses that were waiting to give them rides to early voting locations. "Everything we fought for in 2008 is on the line in 2012," he said.
Mr Romney focused on the Democratic bastion of Cuyahoga County to the north. "It's time for him to leave the White House," he said of Mr Obama at an evening rally in Cuyahoga Falls. "Ohio's going to elect me the next president of the United States."
Republicans credit Mr Romney's strong debate appearance last week as the reason for an increase in national polling. His advisers say they are seeing evidence of that in the swing states most likely to decide the election, Ohio among them.