WASHINGTON — A federal judge has dealt a blow to the prosecution of two former pro-Israel lobbyists by ruling that the government cannot seek to prove that a document key to the case was classified.

In an opinion issued Friday, Judge Thomas Ellis III rejected a bid by prosecutors to prove that in June 2003 one of the lobbyists, Keith Weissman, asked a Pentagon analyst who has pleaded guilty in the case, Lawrence Franklin, to obtain a classified document and suggested it was available from the CIA.

“Significantly, this is the only overt act in which one of the defendants is alleged to have requested a document from a government official,” Judge Ellis wrote. The judge, who sits in Alexandria, Va., gave scant details about the document, although at one point he indicated that it originated with an unnamed foreign government. Some information in the case relates to Iranian influence over attacks on American troops in Iraq.

The indictment filed last year against Mr. Weissman and another lobbyist at the American Israel Public Affairs Committee, Steven Rosen, said the document in question was not classified. Judge Ellis said prosecutors were belatedly attempting to amend the indictment.

“Permitting the government to prove that the document … was classified materially alters an essential fact alleged in the superseding indictment and broaden the bases for conviction,” he wrote. “It is therefore error per se.”

Earlier last week, Judge Ellis issued a long-awaited ruling rejecting the defense’s request that the case be thrown out on constitutional grounds.