Martin H. Simon/ABC

With the first presidential debate less than three weeks away, Mitt Romney is spending lots of time getting ready behind closed doors. In his first comments on that debate prep, he told me that Sen. Rob Portman is a tough stand-in for a president who basically lies in debates.

"I think the challenge that I'll have in the debate is that the president tends to, how shall I say it, to say things that aren't true," Romney said. "I've looked at prior debates. And in that kind of case, it's difficult to say, 'Well, am I going to spend my time correcting things that aren't quite accurate? Or am I going to spend my time talking about the things I want to talk about?"

The former governor told me he's tempted to use Ronald Reagan's famous line against President Carter in a 1980 debate, "There you go again" - the same line that Bill Clinton turned on Romney and the Republicans at the Democratic convention in Charlotte.

When I asked if Portman was crushing him in early debate preps, Romney was coy.

"I'm not revealing those kind of secrets. But I will never debate Rob Portman again," Romney said with a laugh.

"He's very good," he said.

[Senator John McCain expressed a similar sentiment after going against Portman in his 2008 debate prep. McCain later told the New York Times, laughing, "I hate him still."]

The debates are at a critical moment in the campaign with the latest polls showing Obama gaining an edge over Romney in the key battleground states.

Romney told me the debates "may well be" the campaign's make or break moment.

"Sometimes there's something big that happens and they become deciding. Other times, it's like, well, nothing really changed. We're in the same spot we were before the debates. I can't predict what'll happen. But I think it'll be revealing one way or the other," he said.

Romney said he wasn't concerned about new polls showing him trailing in Virginia and Ohio - even though it's virtually impossible for him to get the 270 electoral votes he needs without victories in those two states.

"Well, I'm ahead in a lot of other states, too. I saw one this morning, ahead in Florida, ahead in North Carolina. Gosh, we're even tied in Wisconsin," Romney told me. "These polls are going to bounce around a lot. I don't pay a lot of attention day to day to which state's up and which one's down. But I believe that when the final decisions are being made by the American people, they're going to ask themselves, "Who do I have confidence in to keep America safe? And who do I believe can get our economy doing what it needs to do?"

Romney said the most important numbers in this election are 23, 47 and 16: "23 million Americans out of work or underemployed, 47 million people on food stamps, $16 trillion in debt. And now the Federal Reserve, it says, " Look, this economy is not going well," he said.

And it's because of those economic conditions some Republican allies are worried. George Will on "This Week" said "If the Republican Party cannot win in this environment, it has to get out of politics and find another business." Laura Ingraham said "If you can't beat Barack Obama with this record, then shut down the party, shut it down."

"Well you know, beating an incumbent is never easy. The president exudes an air of likability and friendliness, which is endearing," Romney told me. "But at the same time, I think people recognize that he has not done the job they expected him to do and that he promised he would do."

The race will be decided by "the people in the middle" who "won't make their mind up until the very, very last moment," the former governor said.

And many of those people - 63% of registered voters - want more details about what a Romney presidency would look like, according to our latest ABC News/ Washington Post poll.

The former governor cited his 59 point - and more than 150 pages - economic plan that he released, but acknowledged that "people aren't going to sit down and read a book."

"So that means that in the speeches I give over the coming weeks I need to lay out some of the principles that were described in that book. And I will in more detail," he said.

Democrats say Romney's plan would cause a $2,000 tax hike on the middle class - something Romney disputes and points to a number of studies that say his plan to cut taxes will not increase the deficit, including one by Harvard professor Martin Feldstein.

Feldstein says Romney's math will work, but he would have to eliminate the home mortgage, charitable, state and local tax deductions for incomes greater than $100,000.

When I pressed Romney on that point, he conceded that he actually hadn't read the Feldstein report that he and Paul Ryan cite on the campaign trail.

"I haven't seen his precise study," he said.

"I said that there are five different studies that point out that we can get to a balanced budget without raising taxes on middle income people. Let me tell you, George, the fundamentals of my tax policy are these. Number one, reduce tax burdens on middle-income people. So no one can say my plan is going to raise taxes on middle-income people, because principle number one is keep the burden down on middle-income taxpayers," he said.

Romney defined middle income as $200,000 to $250,000 a year and less.

"Number two, don't reduce the share of taxes paid by the wealthiest. The top 5 percent will still pay the same share of taxes they pay today. That's principle one, principle two. Principle three is create incentives for growth, make it easier for businesses to start and to add jobs. And finally, simplify the code, make it easier for people to pay their taxes than the way they have to now," he said.

When I asked if he and his running mate, Rep. Paul Ryan, would specify which loopholes they would close in order to pay for the tax cuts, Romney said he's relying on his experience as Massachusetts governor.

"I've found that you have to work with the people across the aisle. My legislature was 87% Democrat. So if I'd have come out and said, "Here this is my bill. This is the way I want it," you'd never get it done," he said. "You lay out your principles. Those are my principles, don't raise taxes on middle-income people, make sure the high-income people pay the same share they're paying today, encourage growth by bringing down rates, and finally simplify the code. Those are my principles. I'll stick with them. And I believe that's going to help get the economy going and grow jobs."

I also asked the presidential candidate about a recent poll number that showed registered voters by a 19% margin would prefer to have dinner with Obama instead of Romney.

So what's dinner like at the Romney house? "Chaotic," he said.

"You'd have grandkids climbing all over you. Probably some food would be thrown from one side of the table to the other by one of my grandkids. It'd be a lot of fun," he said.

"So I can't tell people who would have more fun at whose table. But I can tell you the president's a person that a lot of people like. I don't dislike him myself and wish him the very best. But I think the American people are looking for someone who has the capacity to help them get good jobs and more take-home pay. And I do," Romney told me.

Editor's note: This blog has been updated to reflect that in responding to a question from ABC's George Stephanopoulos, Mitt Romney defined middle income for Americans as $200,000-$250,000 and less.