With less than three weeks before the first presidential debate, Mitt Romney says he already knows what his biggest hurdle will be in his three October showdowns with President Barack Obama.

"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 in an interview that aired Friday on ABC's "Good Morning America."

Romney, who spent several days in debate prep last week as the Democratic National Convention was under way, said he's been studying Obama's performance at previous debates. The Republican presidential nominee added he's still trying to outline his strategy.

"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?'" he said. "That's a judgment you make."

The first presidential debate will focus on domestic policy and take place at the University of Denver in Colorado on Oct. 3. The two rivals will face off twice more that same month, including a town hall style debate hosted by CNN Chief Political Correspondent Candy Crowley, anchor of CNN's "State of the Union," on Oct. 16.

Sen. Rob Portman of Ohio, who was on Romney's running mate short list, was tapped to play the role of Obama in debate practice for Romney. Portman also assumed the same role for Sen. John McCain for the GOP nominee's 2008 run against Obama.

Asked if Romney had a difficult time in his first couple of rounds of preparation, Romney said he would not reveal "those kind of secrets."

"But I will never debate Rob Portman again," Romney said, laughing.

Also in the interview, Romney conceded a rare comparison between him and his opponent, saying the two would draw the same "red line" on nuclear weapons in Iran.

"My red line is Iran may not have a nuclear weapon. It is inappropriate for them to have the capacity to terrorize the world," Romney said. "Iran as a nuclear nation is unacceptable to the United States of America."

The president has used almost identical language to describe the situation in Iran. When asked if the two would have the same "red line" going forward, Romney said "yes."

"And recognize that when one says that it's unacceptable to the United States of America that that means what it says. You'll take any action necessary to prevent that development, which is Iran becoming nuclear," Romney said.

His comments come after Israeli Prime Minister Benjamin Netanyahu expressed recent concerns that the international community was not drawing clear "red lines" against Iran.

The Obama administration has pushed back on reports that the White House turned down requests for a meeting from Netanyahu to discuss, in person, threats to Israel by Iran's nuclear program. Obama and Netanyahu held a one-hour phone call Tuesday night to hash out those concerns.

Romney reiterated in the interview, however, that his policies would have been more effective than those of the president in halting efforts to develop nuclear weapons, namely the implementation of harsh sanctions-measures which the administration and other countries have already begun to enforce.

"I said that crippling sanctions needed to be put in place immediately. This was a long time ago, several- five years ago, I believe. Crippling sanctions such that their economy would be on its knees, at this point," he said.