Poor Kent Hance. His face has broken into a rash again and even visit to the Oval Office didn't seem to help.

"I've stuck my neck way out there," the Democratic congressman from Texas who is cosponsoring President Reagan's tax bill said last week, before heading to Camp David to discuss the bill this weekend. "yi don't like being on the other side."

The last time Hance broke out in a nervous rash was six weeks ago when he defected to the Republican side, deserting his fellow Democrats in Congress and risking his political future to lead what he hoped would be a southern conservative stampede to endorse the president's tax package.

"Making that decision was a traumatic experience," he recalls.

Now, with Wednesday's forthcoming House vote too close to call, the lanky Texan is plainly worried. The Democrats have loaded their bill with sweeteners to appeal to oil-state and business-oriented conservatives and, although the administratin has responded in kind, it is still questionable whether Reagan can attract enough Democrats to achieve the kind of dramatic victory on taxes that he managed on budget cuts.

Much has been written of late about the "boll weevils," those 40-odd southern Democrasts who are said to hold the balance of power in the House of Repreentatives. But the story of Hance, until just recently an obscure second-term congressman from Lubbock, indicates that life in the limelight isn't always as comfortable as it seems.

A slender, balding man with an eager smile and a nimble wit, Hance, 38, won a seat on the tax-writing Ways and Means Committee this year by portraying himself to House Democratic leaders as a team player. When he turned around and lent his name to the Reagan tax bill before the Democrats had drafted their own legislation, fellow party members wree furious.

"Disloyal, perfidious, treacherous apostates," fumed Rep. Mickey Leland (D-Tex.) of Hance and his colleague, Phil Gramm, who sponsored Reagan's budget bill. Leland and several other Democrats asked that Hance and Gramm be stripped of their committee assignments. Some Democrats have snubbed Hance in the elevator.

Ways and Means Chairman Dan Rostenkowski (D-Ill.), who has excluded Hance from Democratic caucuses and studiously ignored him in committee of late, is reportedly plotting revenge. "There's no question his own chairman will find ways to stick it to him," said a high-level staffer. h

Worse, Hance has alienated his most powerful aptron, Majority Leader James C. Wright Jr. of Fort Worth, who helped get him the Ways and Means seat so important to Hance's oil constituents.

"I'm personally hurt because I like Kent," Wright said. "He's a cute kid. But he made a deal without even waiting to see what the Democrats would have in their bill. He became the White House patsy on the committee. He didn't have enough stamina to put up with the pressure."

Nor has Hance exactly been welcomed into the Republican fold. Colleagues say he has had limited influence on the bill to which he attached his name, although he has met frequently with the White House and Treaasury officials in charge.

His relations with his cosponsor, New York Rep. Barber B. Conable Jr., ranking Republican on Ways and Means, are distant. No wonder, since Conable had objected to Hance's appointment to the comittee from the outset, calling him a "knee-jerk oil man."

But in the dusty plains of the Texas Panhandle, in towns like Dimmitt, where Hance was born and where, he says, "the highway ends and the West begins," the congressman is a hero.

Son of a rural mail carrier and a secretary, cocaptain of the Dimmitt Bobcats basketball team, a man who is still married to his high-school sweetheart, he became a Lubbock attorney and made it to the state Senate.

Who would have thought that after two years in Congress he'd rise from the shadows of the subcommittee on cotton, livestock and grain to stand next to the president in the Rose Garden, attacking his name to the biggest tax bill in history?

"It wasn't planned," Hance insists, maintaining that he tried to encourage Democratic leaders to compromise with the White House for weeks, and that, only when it became clearly impossible, did he agree to cosponsore the Republican bill.

Although his colleagues suggest it was hunger for publicity and status that persuaded him, Hance depicts his decision as straightforwardly political, the natural result of being elected from a district that voted 72 percent for Reagan.

"I'd go home to town hall meetings in places like Muleshoe and I'd look at people in the eyes and they were saying give the president a chance," said Hance in his West Texas twang. Wright suggests that wealthy oil men, contacted by the White House, might have had more to do with Hance's conversion than the folks in Muleshoe, but Hance denies any such pressure.

His district mail is running 5 to 1 in favor of his stand. Praising Hance's "statesmanship of the highest order," the Lamesa Press-Reporter editorialized, "How many times can you recall a Democratic congressman from far West Texas bucking the entrenched old-line leadership of his party in the interest of what he feels is best for the American people?"

Nonetheless, for a politician who told the Lubbock paper that his first goal was "to lay a firm foundation with my colleagues" and his long-term objective was "moving up the ladder to a leadership position," it is difficult to be, for the moment, a man with a party.

He is taking it with good humor, making light of his nervous rash with reporters and joking to his colleague William M. Brodhead (D-Mich.), who set up a task force to look into discipling defectors, "I see you appointed the lynch mob and I'm glad you put some of my friends on there." y

Indeed, although many of his colleagues feel betrayed at the moment, Hance, by virtue of his outgoing nature and sense of humor, has a reservoir of good will among fellow Democrats and, in the end, he may be forgiven.

Many of them compare him to the more flamboyant Gramm, a conservative ideologue who seems to delight in embarrassing his Democratic colleagues."Kent is weak," said one member of the Democratic leadership. "Gramm is venal."

And Wright said, "I'm not going to punish Kent. I want to redeem him."

Once this tax busines is over, Hance will clearly be ready for redemption. He wouldn't dream of becoming a Republican, he said. "I represent Democrats. I'm a good Democrat. I think the Democratic Party can best apeak for the working people." (The Republican Hance beat to gain his seat in 1980 was George W. Bush, son of the vice president.)

Hance acknowledges his standing in the House has been damaged, but asserts it is "temporary. There'll be other days and other issues. I'm sure I'll have a different position from the president on the farm bill, on the Voting Rights Act, on Social Security."

Sometimes the conservative Texas seems half puzzled at his predicament.Last week he mused: "What's a nice guy like me doing here?"