WASHINGTON — John McCain clinched the Republican presidential nomination Tuesday, an extraordinary comeback for a candidate whose White House hopes were dashed eight years ago and whose second bid was left for dead eight months ago.

"The most important race begins," he said in an Associated Press interview.

According to the AP count, the four-term Arizona senator surpassed the requisite 1,191 GOP delegates as voters in Ohio, Vermont, Rhode Island and Texas put him over the threshold. The triumph came one month after his Super Tuesday coast-to-coast victories gave him an insurmountable lead in the delegate hunt and forced his chief rival, Mitt Romney, to drop out of the race.

"It's a very humbling thing, and I say that with all sincerity," McCain said of finally clinching the nomination.

McCain was heading to the White House on Wednesday for lunch with President Bush _ and an endorsement. The two will make a joint statement afterward.

"The president has said he looks forward to vigorously campaigning for the GOP and tonight it has become clear that the GOP nominee will be Senator John McCain," said White House press secretary Dana Perino. "Of course the president is going to endorse the GOP nominee which is going to be Senator John McCain."

Republicans won't officially nominate McCain until early September at the GOP's national convention in Minneapolis-St. Paul.

Shortly after AP called the race, his chief remaining rival Mike Huckabee withdrew from the race.

In Irving, Texas, the former Arkansas governor praised McCain and said: "My commitment to him and the party is to do everything possible to unite our party but more important to unite our country so that we can be the best we can be."

The general election campaign for the Republican nominee-in-waiting starts now even though Democrats still haven't chosen a candidate. Sens. Barack Obama and Hillary Rodham Clinton continue a protracted battle for their party's nod, leaving McCain an opportunity to unify his party.

"The big battle's to come," he said of the general election. "I do not underestimate the significance nor the size of the challenge."

McCain added: "There are going to be stark choices between a liberal Democrat and a conservative Republican. I believe I can prevail in that contest of ideas and vision."

Asked if he'd leave the Senate to campaign, he said, "I have no intention of resigning from the Senate, but I will discuss it more."

In a Dallas hotel ballroom where McCain addressed supporters, workers hoisted a five-foot-tall banner reading "1191."

"Our campaign must be, and will be more than another tired debate of false promises, empty sound bites, or useless arguments from the past that address not a single American's concerns for their family's security," said McCain, who earlier took a call of congratulations from Clinton and Obama. A spokesman for the Illinois Democrat said he told McCain "he looked forward to running against him in the fall."

After racking up wins in states across the country, McCain entered Tuesday's contests with 1,014 delegates, 177 short of what he needed. McCain won all 17 delegates in Vermont, and at least 69 in Texas, 58 in Ohio and nine in Rhode Island, according to early returns. McCain also picked up about 30 endorsements from party leaders who will automatically attend the convention.

The delegate milestone effectively ends the bruising GOP primary fight that began just days after the November 2006 congressional elections when a slew of Republicans launched candidacies to succeed Bush as the party's standard-bearer and president. At one point, the crowded field reached a dozen.

Only Texas Rep. Ron Paul remains but it's now impossible for him to become the nominee. He has not indicated when he will concede but his departure is inevitable.

McCain's feat caps a remarkable turnaround for the 71-year-old man who began running for president roughly a decade ago when he plotted a bid to overtake Bush, the then-Texas governor and establishment favorite. Back then, the Republican with a long reputation of bucking the party shocked Bush and much of the GOP with his come-from-nowhere double-digit win in New Hampshire. The race turned nasty as it moved to South Carolina, and McCain's bid never recovered from a loss there.

Nonetheless, that campaign put McCain _ already somewhat known because of his Vietnam war-hero biography _ on the national political map and set the stage for his campaign sequel.

Over the next few years, McCain sought to mend his relationship with the Bush political machine and conservatives who make up a cornerstone of the party. He embraced the president and campaigned for him during his successful re-election bid. He also reached out to the party's right-flank and its leaders like the late Jerry Falwell and Pat Robertson who he once derided.

McCain also laid the groundwork for his second White House campaign.

He melded veteran Bush operatives with McCain loyalists from 2000 to build an unrivaled _ and gigantic _ national campaign organization. The loser in 2000, he cast himself as the inevitable nominee in a GOP that historically has nominated the next in line, and the only Republican who could unite a wayward party reeling from a 2006 thumping that put Democrats back in control of Congress.

But staff infighting and financial troubles quickly rocked the campaign. Money was spent faster than collected, and fundraising targets were not met. Top aides vied for primacy. Longtime McCain aides clashed with one-time Bush aides. All that led to a major staff overhaul and an empty bank account _ a near unraveling _ last summer.

By July, the campaign had blown through nearly all of the $25 million it had raised, and McCain had accepted the resignations of two top aides and promoted a third to manage what was left of the campaign; money troubles meant dozens of layoffs while loyalty to the departed aides prompted others to flee.

He took a hit, too, politically with his embrace of the Iraq war that independents opposed and comprehensive immigration reform that conservatives detested. As a result, his standing in polls dropped and fundraising dried up.

Determined to press on, McCain basically started from scratch.

He mapped out a long-shot road ahead with a one-state strategy, hoping he could still emerge as the last man standing if the GOP field remained fractured in part because the influential conservative wing had not rallied around a candidate.

Out of options and short on cash, he turned again to New Hampshire, which viewed him as almost a native son given his attention in 2000.

New Hampshire ended up delivering again, and a victory there led to hard-fought wins in South Carolina, Florida, a slew of delegate-rich states on Super Tuesday Feb. 5, and, ultimately, the nomination.