Senate Democrats, for the first time in history, held closed secret-ballot votes yesterday to choose the chairmen of the senate legislative and study committees.

All the nominees suggested by the Democrat Steering Committee were elected by huge margins, but the taking of secret ballots, heavily lobbied for by Common Cause, a citizens lobby, marked a new departure in procedure in the Caucus of all 62 Democratic senators.

The closest votes came on Russell B. Long (La.) for re-election to Finance Committee chairman, and John L. McClellan (Ark.) for re-election to Appropriations Committee Chairman. In each case, the vote was 42 to 6.

Common Cause had hoped to roudn up more votes against Long as a signal that senators want him to change legislation procedures in his commitee and pave the way for tax reform, but Long said, "I guess Common Cause didn't have all the support it thought."

Fred Wertheimer, Common Cause spokesman, said, "We're happy that they put the new process into effect. It's a healthy thing to have secret ballots."

The new rules on secret ballots were installed two years ago on a motion by Dick Clark (D-Iowa), but never used before. They provide that if a fifth of the caucusmembers present indicate on a secret sheet that they want secret ballots will be held. Yesterday a dozen Democrats out of 50 present demanded secret ballots on all the chairmen.

Majority Leader Robert C. Byrd (D-W.Va.) announced the following results in addition to long and McClellan: Herman E. Talmadge (Ga.), Agriculture, 46 to 2; John C. Stennis (Miss.), Armed Services, 43 to 5; John J. Sparkman (Ala.), Foreign Relations, 44 to 4; James O. Eastland (Miss.), Judiciary, 44 to 4; Warren G. Magnuson (wash.), Commerce, 46 to 2; Jennings Randolph (W. Va.), Enviroment, 46 to 2; Howard W. Cannon (Nev.), Rules, 45 to 3; Adlai E. Stevenson (Ill.), Ethics, 47 to 1; William Proxmire (Wis.), Banking, 45 to 3; Harrison A. Williamson (N.J.), Human Resources, 47 to 1; Frank Church, (Idaho), Aging, 47 to 1; Henry M. Jackson (Wash.), Energy, 46 to 2.

Unanimously elected were Edmund S. Muskie (Maine), Budget; James Abourezk (S. D.), Indians; AbJames A. Ribicoff (Conn.), Governmental Affairs; Alan Cranston (Calif.), Veterans; Daniel K. Inouye (Hawaii), Intelligence, Gaylord Nelson (Wis.), Small Business.

Earlier, the caucus added conservative James B. Allen (Ala.) and, to balance him, liberal John C. Culver (Iowa) to the Judiciary Committee without dropping anyone. Allen previously had declined election when Byrd offered to get off to make way for him.