WASHINGTON (AP) -- Directly challenging the Pentagon's top leadership, Arizona Republican Sen. John McCain on Thursday snubbed a military study on gays as flawed and said letting gays serve openly would be dangerous in a time of war. The Skanner News Video: McCain and Mullen go Head to head

McCain's opposition foreshadows the upcoming Senate debate on a bill that would overturn the 1993 "don't ask, don't tell" law, which bans gays from serving openly in the service.

Senate Majority Leader Harry Reid, D-Nev., has promised a vote, but McCain has helped to block previous debate on the Senate floor.

Further dimming chances of repeal this month was a recent agreement among Senate Republicans not to vote on any bill before addressing tax cuts and government spending.

McCain, a former Navy pilot, comes from a long family line of service in the military. He was a Vietnam era prisoner of war. and was the Republican presidential nominee in 2008 who lost to Barack Obama.

Advocates of repeal had hoped that this week's Pentagon study would have lessened GOP resistance to the bill. The study found that the overwhelming majority of troops were not against seeing the policy repealed.

But among those who did care, most were troops performing combat arms duties. Nearly 60 percent of those in the Marine Corps and in Army combat units said they thought repealing the law would hurt their units' ability to fight on the battlefield.

McCain seized on this finding to argue that forcing such a substantial personnel policy change in a time of war would be wrong for the military and the country. He also criticized the study for scrutinizing only how the law could be repealed, instead of whether doing so would benefit the military.

"At this time, we should be inherently cautious about making any changes that would affect our military, and what changes we do make should be the product of careful and deliberate consideration," McCain said.

McCain's statement was directly challenged by Defense Secretary Robert Gates and Adm. Mike Mullen, the military's top uniformed officer who chairs the Joint Chiefs of Staff.

"Repeal of the law will not prove unacceptable risk to military readiness," Mullen told the Senate Armed Services Committee. "Unit cohesion will not suffer if our units are well-led. And families will not encourage their loved ones to leave the service in droves."

Mullen also said that Congress should act before the courts do, and that wartime is an ideal time for repeal.

"War does not stifle change; it demands it," he said. "It does not make it harder; it facilitates it."

McCain has previously suggested that Mullen's opinion didn't matter as much as other military commanders because he doesn't directly lead troops.

In his opening statement, Mullen seemed to issue a direct challenge to McCain.

"For more than 40 years, I have made decisions that affected and even risked the lives of young men and women," Mullen said. "You do not have to agree with me on this issue. But don't think for one moment that I haven't carefully considered the impact of the advice I give on those who will have to live with the decisions that advice informs."

Marine Gen. James Cartwright, the No. 2 officer on the Joint Chiefs of Staff, said in an interview that if Congress fails to act the military could handle an abrupt about-face mandated by the courts.

He like the other Pentagon leaders said that is by far the second choice, and would be disruptive for forces currently cycling through the military's tightly planned rotation for wartime deployment.

"Bringing this into force quickly means that we have to do some of this in the battlefield. Probably doable, but it's a bigger challenge than we really want to have to take," Cartwright said.

Cartwright and the military chiefs of each service will testify before the same Senate panel on Friday. The focus will be on Marine Commandant Gen. James Amos because of the survey results showing high opposition to repeal among Marine combat troops.

"I cannot speak for him but I will speak as a Marine," Cartwright said. "If the law is repealed the Marine Corps will lead the education, training, and bringing it in," he said. "They will comply with the law, no doubt about it, and they will comply with the law aggressively."