WASHINGTON: The presidential candidates bear down Tuesday on Ohio, a state that could decide the Nov. 6 election, as the US presidential race moves into its final four weeks with the Republican challenger Mitt Romney’s shift toward more moderate positions apparently narrowing the gap with President Barack Obama.

Obama has spent much of his time since last week’s first presidential debate trying to convince voters that Romney has suddenly and fundamentally shifted his position on income taxes. Romney had been promising to lower tax rates for all Americans, something independent experts predicted that would vastly increase the already massive US deficit unless Romney also increased the tax burden for middle income earners.

In the debate, Romney unexpectedly changed course, saying he would not institute any tax cut that raised the deficit or put a greater burden on the middle class. He also presented himself as aggressive and competent during the debate, while Obama put in a poor performance.

The shift in the polls toward Romney after that first confrontation with Obama would seem to reinforce the argument that the candidate who can win voter confidence on fixing the struggling US economy will win the White House.

The debate win notched in his favor, Romney on Monday delivered what his campaign characterized as a major foreign policy speech that attacked Obama on issues where polling shows Obama is seen as clearly viewed as the most capable candidate. Romney sought to burnish his credentials as a potential commander in chief in his address before Virginia Military Institute cadets, asserting that Obama’s efforts have been weak in the volatile Middle East and his leadership in world affairs lacking overall.

Tuesday’s focus on Ohio underscores the importance both men place on the swing state. Romney will be joined in Akron by New Jersey’s tough-talking Gov. Chris Christie. Obama will rally students at Ohio State University in Columbus, the state capital. Ohio is one of a handful of US states where voters do not reliably vote for the presidential candidate of one party or the other. Because the president is not chosen according to the outcome of the nationwide popular vote but in state-by-state contests, Ohio and other so-called swing states will determine the outcome of the election. Romney also plans a stop in Iowa, another swing state, on Tuesday.

Both men were scrambling Monday to show their optimism and determination to win the election.

“I very much intend to win this election,” Obama told donors in San Francisco on Monday night. “But we’re only going to do it if everybody is almost obsessive for the next 29 days.” Romney, speaking on a rainy day in Newport News, Virginia, joined the kind of die-hard supporters he needs for victory. “People wonder why it is I’m so confident we’re going to win,” he told them. “I’m confident because I see you here on a day like this. This is unbelievable.”

As negative ads blanketed the toss-up states, the Obama campaign on Tuesday unleashed one on national broadcast and cable networks featuring its favorite new weapon — Big Bird.

Employing ominous narration, the spot ridicules Romney for singling out the “Sesame Street” character and PBS subsidies as examples of how he would cut spending. “One man has the guts to say his name,” says the ad, flashing to Romney and then the feathered creature. “Big. Yellow. A menace to our economy. Mitt Romney knows it’s not Wall Street you have to worry about. It’s Sesame Street.” The US government partially funds the public network, a favorite target of conservative politicians.

While Obama maintains more paths to victory in the swing states, polling shows a tightening race after more than 67 million people watched Romney shine in the Denver debate last week. The challenger’s path victory is extremely narrow, particularly without Ohio. No Republican has won the presidency without carrying the state.

Based on the presumed outcome of the 41 non-battleground states and Washington, D.C., Obama enters the final period banking on 237 electoral votes. Romney is assured of 191.

On the road to 270, the battleground states account for the final 110 electoral votes: Ohio, Florida, Iowa, Nevada, Wisconsin, New Hampshire, Virginia, North Carolina and Colorado.

Both Democrats and Republicans say internal campaign surveys following last week’s debate show Romney cut into the lead Obama had built up in many key battleground states. But they say Obama still has an advantage in most of them.

A lack of independent polling makes it difficult to know whether that’s true. Romney pulled ahead of Obama, 49 to 45 percent nationally, among likely voters in a Pew Research Center poll conducted after the debate.