DEARBORN, Michigan (CNN) -- All eyes were on Fred Thompson in his GOP presidential debate debut, but the most heated exchanges were between Rudy Giuliani and Mitt Romney in attacks on each other's fiscal records.
Nine Republican presidential candidates share a photo-op moment before the debate Tuesday night.
Tuesday night's debate, Thompson's first since officially joining the race last month, involved nine GOP candidates and touched on trade, taxes and a range of economic and security issues.
"I've got to admit, it was getting a little boring without me, but I'm glad to be here now," Thompson said.
The former U.S. senator from Tennessee was largely spared direct fire from the other candidates in the debate, which CNBC and The Wall Street Journal sponsored.
When asked by moderator Chris Matthews of MSNBC to outline the differences between himself and Romney, Giuliani said his opponent failed to control taxes in Massachusetts. Watch Giuliani spar with Romney »
"I brought taxes down by 17 percent. Under him, taxes went up 11 percent per capita," said Giuliani, the front-runner in most national polls. "I led; he lagged."
Romney replied that he lowered taxes and said state spending in Massachusetts grew at a slower rate while he was governor than New York's budget did during Giuliani's tenure.
The other topic that sparked fireworks was a provocative, albeit hypothetical, point of constitutional interpretation -- would the U.S. president need Congress' permission before launching an attack on Iran's nuclear facilities?
Responding first, Romney said as president, "you sit down with your attorneys" to determine whether such authorization is needed, but he said, "Obviously, the president of the United States has to do what's in the best interest of the United States to protect us against a potential threat."
Romney's answer drew an incredulous retort from Rep. Ron Paul of Texas, who said the president would "absolutely" need Congress' OK before striking Iran.
"This idea of going and talking to attorneys totally baffles me. Why don't we just open up the Constitution and read it?" Paul said. "You're not allowed to go to war without a declaration of war."
However, the panel's general consensus was that the president should be able to launch an attack without authorization if the circumstances called for immediate action, but that he or she should go to Congress if time permits.
"If you have a very narrow window to hit a target, the president's going to have to take that on his shoulders," said Rep. Duncan Hunter of California. "He has the right to do that under the Constitution as the commander in chief."
Sen. John McCain of Arizona agreed that a president would have to move on a threat requiring immediate action, but "if it's a long series of buildups, where the threat becomes greater and greater, of course you want to go to Congress."
McCain added, somewhat cryptically, "I believe that this is a possibility that is maybe closer to reality than we are discussing tonight."
Thompson received one slight barb when Romney said how much the Republican debates reminded him of "Law & Order," the TV series in which Thompson starred.
"It has a huge cast, the series seems to go on forever, and Fred Thompson shows up at the end," Romney said.
Thompson responded, "And to think I thought I was going to be the best actor on the stage."
Thompson -- who has been fashioning himself as a straight-talking, outside-the-Beltway reformer despite years spent as a lawmaker and lobbyist -- said he doesn't think "anybody believes anything coming out of Washington anymore."
He pushed a policy proposal likely to be controversial -- indexing increases in Social Security benefits to the rate of inflation, rather than growth in wages, for future retirees as a way to alleviate financial pressures on the retirement system.
"It wouldn't affect current or near retirement people, but for future retirees, instead of having nothing -- which is what they're headed for under the current situation that's unsustainable -- they would have protection," he said. "It would solve the problem for several years. It wouldn't solve it indefinitely, but it would give us a window of opportunity to get our arms around the problem."
Indexing Social Security to inflation rather than wages is strongly opposed by many Democrats, who argue it will result in sharply lower benefits.
Touching briefly on Iraq, Thompson offered limited criticism of the Bush administration's conduct of the war, saying, "We didn't go in with enough troops, and we didn't know what to expect when we got there."
"But now we're showing signs of progress," he said. "I think we've got to take advantage of the opportunities that we have there, now that we see a window of opportunity for things to turn around and us to stabilize that place and not have to leave with our tail between our legs."
At the end of his two-hour debut, Thompson was asked how the experience felt.
"Just like home," he said, drawing laughter. Then he added, "I didn't say which kind of home." E-mail to a friend