A group of younger House Democrats is preparing to launch a coup against Rep. Melvin Price (D-Ill.), the 79-year-old chairman of the House Armed Services Committee, who they feel is too feeble to continue in the job and has let the panel flounder.

House Democrats are scheduled to caucus Jan. 4 to elect committee chairmen. The top committee posts generally are filled on the basis of seniority and coups against chairmen have been rare.

Democrats said yesterday that Price would be difficult to unseat, despite growing sentiment, especially among less senior Democrats, that the committee needs stronger leadership and a more aggressive posture in dealing with the Reagan administration and the Republican-controlled Senate.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) has indicated that he will work to protect Price. And Price has agreed to a plan, intended to prevent a coup, in which he would step down from the chairmanship at the end of the 99th Congress in two years. Price immediately would give up his chairmanship of the Armed Services subcommittee on research and development.

Price sent a letter to O'Neill this week officially notifying the speaker that he would go along with the plan. Price was unavailable for comment yesterday, but an aide confirmed that the letter had been sent and said Price has been assured of the speaker's support.

Rep. Les Aspin (D-Wis.), a member of the Armed Services Committee, began notifying Democratic lawmakers this week that he may seek the chairmanship, several Democrats said yesterday. A group of conservative Democrats on and off the committee also is lobbying colleagues to replace Price with Aspin or Rep. Bill Nichols (D-Ala.), another committee member.

Nichols is said to be supporting Price but apparently would be interested in becoming chairman if Price were defeated by the caucus, sources said.

Rep. Charles E. Bennett (D-Fla.), who under seniority rules would get the chairmanship after Price, has called lawmakers this week to say he is supporting Price but would run for chairman if Price is not accepted by the caucus.

"It's nothing personal against Price," said one Democrat, who has been lobbying for his ouster. "It's just a question of having some leadership . . . of who's the most effective leader for the committee during a very important time."

"Mel Price's time has passed, I think that's obvious, and I think he knows it," said one committee Democrat, who helped work out the compromise. "But let's find a reasonable way."

Democrats will control the new House by at least 69 votes and therefore get to select committee leaders.

Price became chairman of the committee as the result of one such coup. In 1975 then-Chairman F. Edward Hebert (D-La.) was voted out by the caucus for being too autocratic, and Price, as the next senior Democrat, was voted in. Price had supported Hebert.

In the last four years the committee was more supportive of the administration's defense buildup and of the defense industry than the rest of the Democratic caucus.

Younger Democrats, both conservative and liberal, have complained that the committee has not been aggressive enough in going after cost overruns and needlessly expensive new technology.

They also have expressed frustration that the committee has not focused on trying to come up with a set of long-term goals and directions for defense but instead has taken a narrower view focusing on individual defense budgets and weapons systems.

Price, who was elected in 1944 and is the second most senior Democrat in the House, has had some health problems in recent years, and it is widely felt on the committee that the ranking Republican, Rep. William L. Dickinson of Alabama, has had a free hand to steer the panel.

Democratic lawmakers and officials said it was unclear whether this would be reason enough for caucus members to demand a new chairman, especially with Price's stated intention of giving up the committee in two years.

Bennett, who has served on Armed Services with Price for 36 years, said yesterday, "I'm saying that if Mr. Price goes down, and I hope he doesn't, I'm a candidate."