Imitation, it is said, is the sincerest form of flattery. But here in North Carolina this spring, Republicans have been so hellbent on imitating Democrats they seem to have overshot flattery and moved headlong into recklessness.

The GOP primary for the U.S. Senate has turned into a cat fight royale, worthy of Democrats at their most ornery.

The taunts and cuts and distortions will stop today, when the state's 807,573 registered Republicans can choose between Rep. James T. Broyhill (R-N.C.), an Old Right economic conservative, and former ambassador to Rumania David Funderburk, a New Right social and cultural conservative, for the nomination to a seat that U.S. Sen. John P. East (R-N.C.) is giving up for health reasons.

The North Carolina primary is one of three today, and in two of them, the action is on the Republican side. In Ohio, former governor James A. Rhodes is seeking a comeback but first must get past two Republican challengers, Senate president Paul Gillmor and State Sen. Paul Pfeifer. Several polls favor Rhodes. No significant races will be run in Indiana, site of the other primary.

Here in North Carolina, Broyhill, 58, is the heavy favorite, as he has been from the start. But he has gotten sufficiently bloodied in the brawl that many Democrats, still dispirited by their party's down-the-ticket 1984 losses, now say they think that former governor Terry Sanford, the likely winner of a low-key Democratic primary, can make a close race of it in the fall.

"It's fun to see the other side do it to themselves for a change," crowed state Democratic executive director Ed Turlington. "There's no question we come out of this whole thing the beneficiary."

In the final 10 days of the GOP primary:

*Funderburk has been running television ads accusing Broyhill of "personally writing the bill" that could bring a nuclear waste repository to North Carolina -- a highly unpopular proposal here. Broyhill was the architect of a bill that was subsequently rewritten in the Senate to open the possibility of an East Coast disposal site, but he is now trying to outlaw any such site. He responded with ads calling the Funderburk charge "just plain ridiculous and untrue . . . . It's time these people got the message that North Carolinians are fed up with these political tricks."

*A group of elected Broyhill supporters accused the National Congressional Club, the political organization that backs Funderburk, of engaging in "political terrorism." "They are the rottenest political organization in the nation," said 11th District GOP Chairman Harold Corbin.

*Lamarr Mooneyham, a key Funderburk supporter in the fundamentalist community and a former president of the North Carolina Moral Majority, called Broyhill an "abortionist." Funderburk dissociated himself from the remark.

*R.E. Carter Wrenn, executive director of the Congressional Club, threatened to continue to run anti-Broyhill ads even after the primary is over, as an exercise in political punishment. Hearing this, Sen. Jesse Helms (R-N.C.), the founder of the club, said people would get "blue in the face" if they held their breath waiting for such postprimary ads. Helms has remained neutral in the primary, despite the club's active support for Funderburk. The next day, Wrenn withdrew his threat.

*Funderburk, 42, has accused Broyhill strategists of pushing a whisper campaign accusing him of avoiding service in the Vietnam war. He says he received normal deferrals both as a family man and a student.

Through all this, Broyhill, scion of a prominent furniture-making family, whose 23 years in the House has earned him the title of "Mr. Republican" in North Carolina, has maintained leads in private polls of 33 and 50 percentage points over his rival. His great advantage is that most registered Republicans here are of the traditional, moderate stripe, and nearly half live in the Piedmont counties he has represented in Congress.

By contrast, the Congressional Club, which has tried to establish itself as kind of shadow GOP over the past dozen years, has always had its greatest appeal to low- and middle-income, rural registered Democrats, known here as "Jessecrats." They cannot vote in the GOP primary.

Still, Broyhill strategists are clearly nervous about the nuclear waste ads -- the only ones they have responded to. "This one could hurt us because it's a scare issue," said Brad Hayes, a political adviser for Broyhill. "The truth is that a site isn't going to come to North Carolina, but once you try explaining that, you probably only stir things up."

Meantime, Democrats believe they can use the nuclear issue in a slightly different way: to portray Broyhill as a businessman's Republican who was doing the bidding of the nuclear industry when he wrote the bill.

In the decade of ascendant Republicanism in this state, the GOP has been able to appeal to populist voters on social issues. Democrats hope that by portraying Broyhill as the candidate of big business, they can recapture that populist vote on old-fashioned, have-versus-have-not economic issues.

Before he makes it to the fall campaign, Sanford must first win his own primary in a 10-candidate field; the question is whether he can reach the 50 percent needed to avoid a costly and potentially divisive runoff election.