Former solicitor general Rex E. Lee, a frequent target of the right when he was arguing the Reagan administration's cases before the Supreme Court, is ready to tangle with conservative activists again.

Lee, a Washington lawyer, has been hired by the American Bar Association to defend a lawsuit challenging the group's role in screening judges on behalf of the White House. The suit, brought by the conservative Washington Legal Foundation, reflects conservative anger over a number of potential nominees who have not made it through the ABA process.

The foundation's suit charges that the ABA screening panel acts as a federal advisory committee and should be forced to hold its meetings in public. Foundation officials say the ABA panel is too liberal and has rejected qualified conservatives for reasons more related to ideology than legal credentials.

The White House is not bound by the ABA's recommendations, but in reality presidents almost never nominate a judicial candidate rated unqualified by the bar association.

When Lee left the Justice Department after President Reagan's first term, he was vilified in print by conservative scholars as insufficiently dedicated to their cause. But he says it is just a coincidence that the ABA hired him to take on his old adversaries.

As for his brief in the lawsuit, which is due Feb. 15, Lee is keeping mum. "We're just formulating our strategy at this point," he said.