Tobacco is Mr. Big in North Carolina, a crop that provides farmers with more than $1 billion a year in income. And anything that smacks of a vote against the flue-cured leaf ranks somewhere up there with treason.

As a result, the state's two Republican senators, Jesse Helms and John P. East, are harvesting a sheaf of trouble for switching their votes on the tax bill that doubled the federal excise tax on cigarettes.

They changed "no" votes to "yes" to help the tax bill squeak through the Senate, and even though they later voted against a final conference version, it was too late. The bill became law anyway.

The new tax has created trauma in North Carolina. But tobacco is suffering from other problems as well. Part of the problem is a mediocre crop, part of the problem is the economy and another part is the higher price supports authorized by new tobacco legislation.

Cigarette companies are nervous about a consumption drop that is projected as high as 8 percent. Tobacco isn't moving well at the big North Carolina auction markets, and more of it than usual is flooding into the government support-loan pool. Export sales have fallen precipitously. Farmers are furious at the turn of events.

"We can't take sides politically," said Bill Little, an official of the state Farm Bureau Federation, "but concern has been expressed by the farm community and we have expressed it to the senators. . . . They have received a lot of flak and they are attempting to explain their votes."

State Democrats, meanwhile, gloatingly united against the tax, are savoring the fallout from last month's congressional action that raised the cigarette tax to 16 cents.

They're looking ahead to the November congressional elections, with most of their seven incumbents facing tough sledding, and to 1984, when Helms may be challenged for his Senate seat by Gov. James B. Hunt Jr.

Hunt says he was "very disappointed" by the tax vote and fears the state economy will be hurt badly as a result--lower state tax collections and fewer jobs and manufacturing activity if cigarette consumption drops as expected.

The Democrats have started a campaign to publicize the Helms-East switcheroo on the tax bill. Within days of the switch, they were being assailed as "The Tobacco Tax Twins" in a full-page ad published by the state party in the Raleigh News and Observer. Now they are papering the state with reprints of the ad, urging an anti-Republican vote in November.

For his part, Helms, chairman of the Senate Agriculture Committee, has sent many constituents a long letter explaining his first vote for the tax as one of the costs of his earlier successful effort to win passage of a new tobacco price-support program.

And East asserts undying loyalty to tobacco, but he has told Carolinians that he switched his vote only after a personal appeal from President Reagan. He said he would help Reagan "fix a flat tire" on that one vote, but he wasn't going "for the whole ride."

An East aide said that critical mail continues to outpace supportive mail from the state. "We're confident that folks in North Carolina will understand that both senators tried very hard to change the tobacco provisions . . . but politically, you never know what will happen," the aide said.

Helms assistant Clint Fuller, talking with the Associated Press, echoed that. "We hope the people will understand it," he said. "Of course in North Carolina tobacco is sacrosanct. . . . You can't say anything or do anything against tobacco without bringing the house down. It's like Social Security on the national level."