John McCain cast his opposition to a troop withdrawal date in Iraq in historic terms Thursday during a FOX News interview, saying he would continue to fight anybody trying to end the war prematurely "every step of the way."

"I think one of seminal events historians will look at in this whole struggle against radical extremism was our ability to beat back the Democrats' effort to set a date for withdrawal, which I believe would have been chaos," McCain said.

In the hour-long interview with FOX News' Sean Hannity, the GOP nominee-in-waiting also firmly defended his conservative credentials and attempted to close the book on a long-standing story that he flirted with joining the Democratic Party years ago.

Though he's come under fire from some in his party for sometimes siding with Democrats on immigration, campaign finance and taxes, McCain defended his votes and said his campaign in the November general election against Hillary Clinton or Barack Obama will be a pitched battle of right vs. left.

"This is a fundamental clash between a liberal Democrat and a conservative Republican," McCain said, declining to respond to Democratic quips that he's running for a third Bush term. "I relish the combat. ... I do not underestimate the challenge."

McCain dismissed reports that he flirted with leaving his party in 2001, when he met with Senate Democratic leaders.

"They asked to meet with me, and I met with them, and I said categorically, 'Absolutely not,'" McCain said in the FOX News interview. "No one arranged it that I know of. The fact is I thought it was incredible. I am a proud Republican conservative and I made that very clear."

Such reports seem a sore subject for McCain. The Arizona senator snapped at a New York Times reporter last week who asked him about his 2004 conversation with Sen. John Kerry on becoming Kerry's running mate on the Democratic presidential ticket.

McCain has been dogged throughout the campaign by conservative leaders and talk show hosts who question his faith in conservative principles, and the clamor rose the closer McCain got to winning enough delegates to lock down the nomination.

On his initial opposition to President Bush's tax cut initiatives, McCain told FOX News he wouldn't recant his vote, explaining the Bush fiscal packages didn't show enough spending restraint. Yet he lamented the continued growth of government and said he'd reject new taxes if he is elected president. He said the current Bush tax cuts should be made permanent.

He wouldn't say whether he'd endorse another version of the immigration reform bill he and Democratic Sen. Ted Kennedy backed, a bill decried as amnesty by many conservatives.

"It's not gonna be there," he said when asked if he'd sign the legislation as president. "The lesson is they want the borders secured first. ... My friend, we failed."

McCain emerged the victor of the GOP primary season after a heated and bitter race with former Massachusetts Gov. Mitt Romney. McCain pointed to the support of his former rivals, including Romney, as evidence the party is rallying behind him, but McCain declined to give any clear indication whom he favors as a running mate.

"(Romney) was very gracious to me. ... He's earned a place in the Republican Party that's very important. Rudy (Giuliani) I've loved for years and I mean it. ... and Governor (Mike) Huckabee has really earned himself -- so we're just starting a process, but obviously there are people who are highly qualified."

Romney said Tuesday on FOX News that he would feel honored if asked to be McCain's running mate.

It isn't clear yet how much the Iraq war will become an issue in the general election, but McCain spoke confidently about his role in the war effort.

"I am proud of the effort some of us were involved in that stopped this effort to set a date for withdrawal," McCain said. "It came very close, it came very close, and a lot of us put everything on the line, because we knew that if there's a date for withdrawal, that's surrender, Al Qaeda wins and there's chaos and genocide, and we're back and they follow us home."

McCain also responded to criticism from the Democratic presidential candidates that he intends to stay in Iraq for "100 years." McCain said he's only talking about "American presence."

"This war will be won if we stay with it, and then it's a question of American presence. We have troops in South Korea as a result of the Korean War, we have troops in Germany and Japan ... so that's an agreement. We have troops in Kuwait as a result of the first Gulf War," he said, adding that nobody is protesting the U.S. troop presence in those countries.