Senator Marco Rubio of Florida struggled on Sunday to give clear answers about whether it was a mistake for the United States to go to war against Iraq in 2003, becoming the latest Republican presidential candidate to trip on the wisdom of the military invasion.

Under a barrage of questions from Chris Wallace of Fox News, Mr. Rubio repeatedly said “it was not a mistake” for President George W. Bush to order the invasion based on the intelligence he had at the time. But Mr. Rubio grew defensive as Mr. Wallace pressed him to say flatly whether he now believed the war was a mistake. Mr. Rubio chose instead to criticize the questions themselves, saying that in “the real world” presidents have to make decisions based on evidence presented to them at the time.

“It’s not a mistake — I still say it was not a mistake because the president was presented with intelligence that said Iraq had weapons of mass destruction, it was governed by a man who had committed atrocities in the past with weapons of mass destruction,” Mr. Rubio said on “Fox News Sunday.”

A moment later, as Mr. Wallace tried to pin him down on his view, Mr. Rubio began to reply, “Based on what we know now, I think everyone agrees — — ” but Mr. Wallace cut him off before he finished the thought.

“So was it a mistake now?” Mr. Wallace asked.

“I don’t understand the question you’re asking,” Mr. Rubio said.

At one point Mr. Rubio, in discussing the importance of hindsight on the Iraq war, raised a recent boxing fight to make a point. “Based on what we know, a lot of things — based on what we know now, I wouldn’t have thought Manny Pacquiao was going to beat, uh, in that fight a couple weeks ago — — ” Mr. Rubio said before Mr. Wallace interrupted.

The back-and-forth resulted in a three-minute video clip that Republican opponents could use against Mr. Rubio in the future, given that he came across as a politician used to debating fine points and nuances in the United States Senate — a problem that then-Senator John Kerry faced in his presidential run in 2004 — rather than as a seasoned leader used to giving clear statements. Mr. Rubio’s readiness for the presidency has been questioned among some Republican voters, given than he is a 43-year-old first-term senator, and moments like the boxing reference seemed discordant on a subject like the Iraq war.

Last week, former Governor Jeb Bush of Florida was the one in the hot seat, giving four different answers to whether he thought President Bush, his brother, made a mistake going to war in Iraq. The former governor ultimately said he would not have ordered the invasion in hindsight given that intelligence about Iraq’s chemical weapons turned out to wrong. Mr. Rubio, in his interview on Sunday, chose not to pile on Mr. Bush.

Another Republican presidential candidate, Senator Rand Paul of Kentucky, said on NBC’s “Meet the Press” on Sunday that it was an “important question” for Mr. Bush and others to say whether “it is a good idea to topple secular dictators, and what happens when we do.”

“I think when Hussein was toppled, we got chaos, and we still have chaos,” Mr. Paul said. He, too, decided not to go after Mr. Bush on his turbulent week of Iraq answers.

A likely Republican candidate, Gov. Scott Walker of Wisconsin, offered some words of defense for President Bush on Sunday rather than critiquing the war or taking a shot at Jeb Bush.

“I think any president, regardless of party, probably would have made a similar decision to what President Bush did at the time with the information he had available,” Mr. Walker said on CBS’s “Face the Nation.” That said, Mr. Walker added that knowing now that the 2003 weapons intelligence was flawed, “we probably wouldn’t have taken that tack” in invading Iraq.

Mr. Walker, who is boning up on national security issues to address some Republican concerns about his preparedness for the presidency, was also asked on CBS about his past comments that the most important foreign policy decision of his lifetime was Ronald Reagan’s decision to fire air traffic controllers because it sent a message to the Soviet Union about his toughness. Mr. Walker, who returned from Israel on Thursday after a trip to meet leaders there and learn more about the Middle East, chose not to change or calibrate his Reagan answer; instead he said that the firing of the controllers was “peace through strength personified.”

On the Democratic side of the presidential race, Senator Bernie Sanders of Vermont called on Hillary Rodham Clinton to take a position on the congressional legislation to grant authority to President Obama to accelerate negotiations over a 12-nation trade deal known as the Trans-Pacific Partnership.

“You can’t be on the fence on this one: You’re either for it or against it,” Mr. Sanders said on CNN’s “State of the Union.” Mr. Sanders opposed the trade authority and has drawn support from liberals for his stances to the left of Mrs. Clinton. “No fence-sitting on this one,” he added.

Mrs. Clinton, who is seeking the Democratic presidential nomination for a second time, has spoken positively about the Trans-Pacific Partnership in the past, but recently declined to endorse or oppose it and instead raised some concerns about the deal.

As for the fate of the trade-negotiating legislation itself, Representative Paul D. Ryan of Wisconsin, chairman of the House Ways and Means Committee, said he was confident that the House would ultimately support it in spite of concerns among many members. “We will have the votes,” Mr. Ryan said.