Supporters of Albert H. Grenadier for a circuit court judgeship in Alexandria said today they have commitments from a "clear majority" of Virginia Senate Democrats to vote for Grenadier over former House majority leader James M. Thomson.

"We have commitments from 18 senators," Sen. Clive. L. DuVal II (D-Fairfax), said in an interview. "We have identified only eight firm votes for Thomson and the other eight are undecided."

Another Senate source cautioned, however, that completely reliable vote counts are difficult to obtain in judgeship elections, which are decided by secret ballots in meeting of the Senate and House of Delegates Democratic caucuses.

Grenadier won the endorsement of the Alexandria Bar Association over Thomson, 140 to 113, after an unusually heated campaign that focused on Thomson's record as an ardent segregationist and foe of ratification of the proposed Equal Rights Amendment in the House of Delegates.

Grenadier also has the backing of the two Democratic legislators representing Alexandria, Dels. Richard R.G. Hobson and Elise B. Heinz. The combination of bar and legislator support usually is decisive in a judgeship contest. Thomson's personal ties to legislators after 22 years in the house, however, have kept his hopes for the judgeship alive.

Neither House nor Senate Democratic leaders have yet set a caucus to choose a successor to Circuit Court Judge Franklin P. Backus, whose retirement becomes effective on Thursday.

Hobson said he is still confident that Grenadier can win the endorsement of the house caucus but now hopes that the 34-member Senate caucus will take its vote first. "I think that would settle the issue," he said.

Sen. Peter K. Babalas (D-Norfolk), who has been canvassing Democratic senators for Thomson, was taken ill today and was not available for comment.