These last, final days, have been wrenchingly painful for George Herbert Walker Bush, the happy warrior of the 1980 presidential race.

He is a man torn between his head and his heart. His advisers are telling him it is time to drop out. The numbers tell him he is finished.

But he does not want to quit. He had invested too much of his time and soul. He has run too long, been written off too many times, been underestimated too long.

His campaign is broke, flat busted. "I need financial support to fight it out in the trenches," he said today. "And that's what I want to do, obviously."

So, he has kept campaigning, cut off from his closest friends and advisers. He has wrestled with what he calls "the toughest decision I've made in my life," in a small room in one of the worst Holiday Inns in America, located in a smelly swamp off the New Jersey Turnpike.

It is the kind of place, a television technician joked, where he walked into a bar and a bouncer stopped him to ask him if he was carrying a knife. "I said no. So the guy gave me one to carry for protection."

Every time Bush ventures from his room, the political realities slap at his face. "George Bush, the loser, just walked by," a man Bush passed on the way to an interview this morning shouted. "Now George Bush May Go Into a Meltdown," blared a New York Post headline.

"You said the opera wasn't over until the fat lady sings," a caller declared in a revealing radio talk show today. "As far as I'm concerned the fat lady sang in the Oregon primary. Why don't you get out?"

"What's wrong with the Rocky syndrome? What's wrong with shoving the fat lady back in the wings," Bush replied testily. "I don't need a lecture from you on that."

His campaign is slowly collapsing around him. Decisions are made in his Alexandria, Va., campaign headquarters. Bush doesn't know about them. No one tells him, until reporters corner him for comment.

When a reporter told him Thursday night that the concensus from a meeting of his congressional supporters was that he should suspend campaigning, he looked like he had been hit over the head with a baseball bat. He turned white, obviously shaken.

"That's the kind of information that's of real interest to me," he sputtered.

But he hears just enough words of encouragement to keep going to one more factory, one more rally, one more interview, carrying out what increasingly appears to be a charade.

He is an eternal optimist, a super achiever all of his 55 years. Asked if he was depressed today, he replied: "No, I'm exhilarated. But I don't like the perception that I may not be able to do what I set out to do."

He has been at this for two years now, day in, day out, week in, week out. No one has run longer or at greater odds. Understandably, he grasps for the thinnest of threads.

"It is funny out there. People don't like what they're being told: that the choice is between Carter and Reagan," he said on WOR radio in New York today. "There's a dynamic at work . . . Who knows what is going to happen between now and Detroit? It's a strange year."

Bush thinks his late-primary wins in the big industrial states of Michigan and Pennsylvania have made a point. "What I think it shows is there is a fluidity, a fludity, a mobility in the electoral process this year," he told a high school crowd in Tenafly, N.J., Thursday night.

"Gov. Reagan has done very, very well and the numbers I face are enormously difficult. But I was able to handle him in the East and demonstrate strength in the Midwest. And it is my perception for a Republican to win next fall, he's got to demonstrate strength in the great industrial Midwest and the Northeast, and we've done it."

Yet Bush steadfastly has refused to attack Reagan with anything but kid gloves. "And I'm not going to start before the denouement," he said today. ". . . Maybe that's why I'm not doing better."

For months, his strategy has relied on defeating Reagan in the last three big state primaries June 3 in Ohio, New Jersey and California. The theory was that this would cause Reagan's 4-to-1 lead in delegates to collapse, and would attract scores of the 49 percent of delegates who are legally unbound.

But on Thursday Bush's campaign abandoned the California contest. And today he conceded Reagan's lead was insurmountable, adding at one point, "The end is in sight."

Late tonight he was scheduled to board a small chartered plane and return home to Houston, where he is to decide over the weekend if he will continue his candidacy. A press conference is scheduled for 10 a.m. Monday to announce his decision.

"I'll be honest with you," he said. "It's a very tough call."