DAYTON — The endgame at hand, President Barack Obama and Republican Mitt Romney plunged into the final two weeks of an excruciatingly close race for the White House Tuesday with TV advertising nearing an astronomical $1 billion and millions of Americans casting early ballots in all regions of the country.

Increasingly, Ohio looms as ground zero in a campaign waged in tough economic times. The state’s unemployment rate of 7 percent is well below the national average of 7.8 percent, Obama has campaigned here more than in any other state and Romney has booked a heavy schedule of appearances in hopes of a breakthrough.

The economy was the theme Tuesday as the two rivals put their final, foreign policy-focused debate behind.

Obama brandished a new 20-page summary of his second-term agenda and told a campaign crowd in Florida his rival’s blueprint “doesn’t really create jobs. His deficit plan doesn’t reduce the deficit; it adds to it.”

More than that, he said Romney changes his positions so often that he can’t be trusted.

In Dayton, Obama said of his rival: “In the closing weeks of the campaign, he’s doing everything he can to hide his true positions. He is terrific at making presentations about stuff he thinks is wrong with America, but he sure can’t give you an answer about what will make it right. And that’s not leadership you can trust.”

Before flying to Ohio for his 17th trip of the election year, Obama also said with a hint of humility: “It doesn’t mean that every candidate is going to get everything done all at once perfectly, but you want somebody to be able to look you in the eye and say, here’s what I believe.”  

Romney countered in an appearance before a large, cheering crowd in Henderson, Nev. He said Obama wants a new term for the same policies that have produced slow economic growth and high unemployment for four long years. “He is a status quo candidate. ... That’s why his campaign is slipping and ours is gaining so much steam,” he said.

Romney’s aides dismissed Obama’s 20-page booklet as nothing new, and the former Massachusetts governor said of the president: “His vision for the future is a repeat of the past.”

There seemed to be no end to the television advertising in a season when voters report they are heartily sick of it.

Material collected by ad trackers showed the two candidates and allied groups have spent or reserved nearly $950 million so far on television commercials, much of it negative, some of it harshly so. Romney and GOP groups had a $100 million advantage over Obama and his supporters, although variations in the purchase price made it difficult to compare the number of ads each side had run.

Increasingly, the two campaigns were focused on turning out their supporters in early balloting under way in more than half the states.

“Every single day right now is Election Day,” Obama’s campaign manager, Jim Messina, told reporters. On that, at least, Republicans offered no rebuttal.

About 5 million voters have already cast ballots according to data collected by the United States Elections Project at George Mason University, and about 35 million are expected to do so before Nov. 6.

While no votes will be counted until Election Day, the group said Democrats have cast more ballots than Republicans in the battleground states of North Carolina and Iowa by about 20 percentage points, while in Nevada, about 121,000 people have voted — 49 percent Democrats and 35 percent Republicans.

Republicans have an early edge in Colorado, where Republicans have cast 43 percent of the 25,000 ballots to date, to 34 percent for Democrats.

Romney’s camp projected confidence as the race entered its final phase, still riding an October surge in the polls that began after the challenger’s dominant performance in the first presidential debate on Oct. 3 in Denver.

The Electoral College math made clear neither man had sealed a victory.

Wins in Ohio and in Wisconsin — a state that Democrats have carried in the past six presidential elections — would leave Obama only five electoral votes short of the 270 needed for victory.

That placed a premium on Ohio — readily apparent from the candidates’ campaign schedules and the millions in television advertising flooding the state.

Romney arrives in Cincinnati on Wednesday night after a Western swing, and is expected to spend all day Thursday and part of Friday campaigning across the state.

His running mate, Paul Ryan, is to deliver a speech on the economy Wednesday at Cleveland State University, something of an unusual event in a campaign with only two weeks to run.

Obama intends to fly into the state Thursday at the end of a two-day, cross-country trip into a half-dozen battlegrounds.

Vice President Joe Biden was in Toledo during the day before heading to Dayton to join Obama for a rally, mid-way through Biden’s three-day tour of the state.

Barring a last-minute change, Obama appears on course to win states and the District of Columbia that account for 237 of the 270 electoral votes needed for victory. Romney has a firm hold on states with 191 electoral votes.

The battlegrounds account for the remaining 110 electoral votes: Florida (29), North Carolina (15), Virginia (13), New Hampshire (4), Iowa (6), Colorado (9), Nevada (6), Ohio (18) and Wisconsin (10).