The cards and letters are pouring in already.

President Reagan has yet to decide how to go about fulfilling his campaign promise to dismantle the Department of Education, but opposing lobbying groups already are gearing up to help, or block, whatever plan the administration comes up with.

From Mobile, Ala., for instance, school board president Dan C. Alexander Jr. and his "Taxpayers Education Lobby" have mounted a direct-mail campaign to, in his words, "build a little backbone in Reagan and his bunch" to carry out the pledge. The attorney is using conservative fund-raiser Richard Viguerie and a letter from Sen. Roger W. Jepsen (R-Iowa) to stir up interest and cash, and his group claims 13,000 members so far.

At the same time, lobbyists for a broad assortment of school groups that fought to create the department are doing some letter-writing of their own to try to save their voice in the Cabinet. Allen S. Cohen, chairman of the Department of Education Coalition, said, "We think we're going to win this one. It took two years of careful debate to get the department passed. A lot of members don't think the department has had a chance yet to prove itself.

"The key for us is that the administration's hidden agenda seems to be to zero out all federal education funding. There won't be any savings other than eliminating programs."

Rep. Jack Brooks (D-Tex.), chairman of the House Government Operations Committee, which will consider any legislation to dismantle the department, echoed that view a month ago after President Reagan reiterated his campaign pledge to kill the one-and-a-half-year-old department. "It is the education programs that cost money," he said. "The only way to save more money is to give the department less to administer. That is what the administration must have in mind and Congress should realize it."

Secretary of Education Terrel H. Bell has said dismantling the department will save only about $140 million. He assured members of a House appropriations subcommittee last week that the administration doesn't intend to cut all federal funding for education.

He did note that federal funds for education amount to only about 8 percent of total spending on education. "I am convinced education is not being cut any more harshly than other departments," he said.

A Bell aide said the secretary and Gary L. Jones, his deputy undersecretary for planning, met with White House counselor Edwin Meese, domestic staff chief Martin Anderson and deputy budget director Ed Harper last week to discuss dismantling options. They are scheduled to have decision memos ready for a Cabinet council by Friday.

Bell has been pushing a plan to transfer his department's big money programs to other agencies, and keep only a research and technical capacity in the form of a foundation, such as the National Science Foundation. The secretary told the House subcommittee he opposes any plan to put federal education programs back into what is now the Health and Human Services Department because, he said, they would get lost in the giant bureaucracy there.

Jepsen's letter contends Bell's four options aren't good enough and urges opponents of the department to sign a petition and send in money to support abolishing the department, through a bill sponsored by Rep. G. William Whitehurst (R-Va.). The petition calls the department "an intrusion on the right of local control or our schools" and says it was created "as a political payoff to the union leaders of the National Education Association who want to control the schools of America."

Earlier this year, an anti-department letter signed by Virginia State Del. John S. Buckley went further. Its petition accused the department of running a sex misinformation program "where the school children of America are being taught that homosexuality is normal and free love is permissible."

Cohen, of the pro-department forces, said such tactics simply help his cause.

The NEA, in the meantime, is preparing fact sheets citing the department's accomplishments. For instance, it says, changing regulation-writing procedures has cut the time it takes to get a regulation approved from 510 to 240 days and the number of offices that must sign off on a reg from 23 to five.

As for the department itself, morale on "the deck of the Titanic," as some call it, has been better. Bell announced the first department firings last week, although they were caused by folding some elementary and secondary programs into block grants, rather than the department's proposed termination. Some career department officials admit they're polishing their resumes.

And when a Reagan political appointee wore a T-shirt around his office sporting the message, "RIFed by Reagan. Out by Christmas," not many folks were amused.