Once McCain gets the 1,191 delegates needed to officially clinch the nomination, the party and its new standard-bearer then must convince a public craving change to keep a Republican in the White House in the midst of a drawn-out Iraq war and a sluggish economy. That's a daunting task by any measure.
But several factors further complicate his run.
Approaching age 72 this year, McCain would be the oldest president ever elected and is certain to face doubts and questions that come with that distinction. His offbeat humor and occasional temper can be grating to even those who know him best. And, his record of breaking with the GOP to work across the aisle irks the conservative wing of the party that he'll need in the fall.
Conversely, the Democratic Party is highly energized and will have a history-making nominee -- either the first female in Hillary Rodham Clinton or the first black in Barack Obama -- calling for a new direction after eight years of Republican rule by the unpopular President Bush.
"With that scenario I'd have to withdraw my nomination," McCain joked to reporters recently on his campaign bus, laughing when one laid out the obstacles facing him and the party. "That scenario's so bleak!"
So it seems.
Becoming serious, McCain insisted he's undeterred.
"I, frankly, like where I am," he said. "I do not try to understate in any way the magnitude of the challenge we face. But I'm confident that we can present the choices in such a way that we can win."
The Arizona senator claims he can make a compelling argument to what he says continues to be a right-of-center country, despite GOP losses at all electoral levels in 2006 as well as a headwind -- and fundraising -- that overwhelmingly favors Democrats.
People, he says, will have a choice between a conservative Republican and a liberal Democrat -- a signal that the coming months will be framed primarily by ideology as he casts his Democratic rivals as big-government, soft-on-security liberals.
He frequently points to hypothetical head-to-head polls that show him in close contention -- if not leading -- Obama or Clinton across the country and claims that his long reputation as a Republican reformer during his 20-plus years in Washington can satisfy the public's call for change.
But on a fundamental level, McCain simply being a Republican makes it difficult to argue that he can put the country on a different path -- as polls show the public clearly wants -- than the Republican president he would succeed and with whom he agrees on big-ticket issues such as Iraq and taxes.
Like Bush, he says troops should stay and the president's tax cuts should be made be permanent.
Already, Democrats are casting him as a continuation of Bush's eight-year reign.
"I don't want ... four more years of George Bush and I think that's what John McCain offers us," says Howard Dean, the Democratic Party chairman.
Ohio, Texas, Rhode Island and Vermont were holding nominating contests Tuesday, and McCain hoped those states gave him enough delegates to seize the nomination. Preliminary exit polls for The Associated Press and television networks showed that independents, McCain's strong suit, accounted for about one in seven GOP voters in Ohio and one in five in Texas.
McCain's expected elevation to GOP nominee -- and upcoming difficult general election campaign -- is a remarkable conclusion to his rocky primary campaign.
After losing his first presidential bid to Bush eight years ago, McCain began his second campaign as the presumed front-runner for the GOP nomination -- and sought early on to position himself as the anointed 2008 nominee in an extraordinarily crowded field.
With no vice president in the running and no obvious heir to Bush, McCain seemed the closest thing to the next in line for the nomination in a party that has a long history of giving the nod to the Republican whose turn is up.
When he started his campaign, he melded veterans of Bush's back-to-back successful elections with his own longtime loyalists to build a behemoth national campaign. He courted the party establishment he had a long record of spurning and there was an all-aboard feel as the McCain train started out of the station in late 2006.
But he ended up squandering any advantage he had then early on. Political, financial and organizational turmoil rocked the campaign over the next six months and left it in tatters.
Nevertheless, he pressed on. He argued that the field was so flawed, that he had as good a chance as any to emerge the nominee.
In the end, he persevered and came back from the brink of political death against all odds.
He hopes he can defy them again in the fall.
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