WASHINGTON -- Sen. John McCain said he wants the military leadership in Afghanistan to use the same aggressive approach that Gen. David Petraeus used successfully in Iraq.

The Arizona Republican said Army Gen. Stanley McChrystal should say exactly how many troops he needs in Afghanistan, let the Congress debate it and President Barack Obama would make the ultimate decision.

Troops in Afghanistan should "clear and hold" an environment for people so that economic and political progress can be made, he said. McCain said he worries McChrystal will be pressured to ask for lower troop totals than he needs.

"I don't think it's necessarily from the president," he said. "I think it's from the people around him and others and that I think don't want to see a significant increase in our troops' presence there."

On the question of what it will take to turn the tide in Afghanistan, McCain said: "I think within a year to 18 months you could start to see progress."

"We're facing a very determined enemy that will stand and fight in some instances that are very adaptable, and obviously with safe havens in Pakistan," he said. "But as the president described it in the campaign, this is a good war and one that we have to win. And I think he'll hold to that."

McCain acknowledged that public opinion on Afghanistan is slipping. But he said that opinion could be reversed.

"I think you need to see a reversal of these very alarming and disturbing trends on attacks, casualties, areas of the country that the Taliban has increased control of."

Even if trends can't be reversed in that amount of time, McCain does not believe the U.S. can leave.

"We always have to remember that we cannot allow Afghanistan to return to a base for terrorist attacks on the United States and our allies," he said. "That's why we went in in the first place."

McCain spoke in an interview for broadcast Sunday on ABC's "This Week."