Sen. Jesse Helms (R-N.C.) has pleaded with Treasury Secretary Donald T. Regan to intervene personally in a tobacco customs case that he said holds the key to his reelection next year.

"I need your help, Don," Helms said in the second of two letters sent last year asking the Treasury secretary to move in on a tariff classification case that traditionally has been decided in a quasi-judicial, non-political basis by the U.S. Customs Service, an arm of Treasury.

Helms wanted Regan's support for a petition by a North Carolina tobacco farmer that would double the tariff on imported tobacco used as filler by American cigarette makers. The petition is opposed by the five major American cigarette makers; the major exporting nations of Brazil and Mexico, and the Leaf Tobacco Export Association, which sells mixed tobacco containing the filler overseas, according to attorneys involved in the case. The petition has been denied by Customs twice before, in 1976 and 1980.

Yesterday, the Customs Service sent its decision on the latest petition to Assistant Secretary John M. Walker Jr., who is charged with making a procedural review. Treasury officials, who declined to reveal what the ruling was, said Walker immediately returned it after noting that all Customs officials involved in the decision had not signed off on it.

The ruling came 15 months after Helms' first letter to Regan and nine months after the second letter, in which the North Carolina senator wrote, "If I am to be helped, it must be done now." The last five words were underlined in the letter, obtained under the Freedom of Information Act by David Busby of Busby, Rehm and Leonard, a law firm representing tobacco interests opposed to the increased tariffs.

"If the 'scrap tobacco' issue is not resolved quickly, the momentum will have been established to take me out of the Senate in 1984," Helms wrote.

Regan advised Helms to have the tobacco farmer refile the petition and promised fast Customs Service action on it, according to a letter Busby obtained. But there is no indication in the correspondence that the Treasury secretary intervened in the case.

Helms, the controversial ultra-conservative chairman of the Senate Agriculture Committee, referred in his letters to two key votes he made supporting the administration.

"You and I discussed the matter last year when my vote was needed on the debt ceiling," Helms wrote Regan after the North Carolina senator, known for his pinchpenny views on domestic federal spending, voted in favor of increasing the debt ceiling past one trillion dollars.

A Treasury spokesman said that Helms presented his views to Regan during that meeting and one on July 8, but that "the secretary made no commitment." Helms and his top aide did not return phone calls.

Helms, facing a tough reelection fight against Democratic Gov. James B. Hunt Jr., wrote in another letter, "My adversaries in North Carolina have seized on the issue of my vote on the tax bill. They are running full page newspaper advertisements, holding press conferences, etc."

Helms helped produce a victory for the administration's 1982 tax bill, which contained a provision opposed by tobacco interests that doubled federal excise taxes on cigarettes to 16 cents a pack.

"I am not an alarmist, and it really doesn't matter to me personally whether I am reelected," Helms added. "I will have been able to serve in the Senate for 12 years, doing the best I could to promote the principles of free enterprise."

Reports that Helms was claiming a Regan commitment for a favorable ruling drew strong protests early this month from Sen. John C. Danforth (R-Mo.), chairman of the Senate Finance Committee's trade subcommittee, and three ranking members of the House Ways and Means Committee.

Regan responded that "neither I, nor any member of my staff, nor any other official of the Treasury Department has communicated with the Customs Service in an effort to influence the outcome of this case."