A federal judge in Alexandria dismissed a suit yesterday by a feminist who claimed she was illegally expelled as treasurer of the Alexandria City Democratic Committee for allegedly helping defeat Virginia Democratic House Leader James M. Thomson in November.

Marianne Wesley Fowler had sued various Democratic committees, claiming that they had violated her constitutional right to free speech, and did not give her proper hearings after her expulsion. U.S. District Judge Oren R. Lewis rejected her claim, saying the dispute was not a matter for the federal courts to decide.

Last December at a hotly debated and bitter meeting, Fowler was removed from the Alexandria committee for allegedly violating party by-laws that forbid any committee member from "affirmatively" supporting "by public actions" any candidate opposed by a Democratic nominee.

Fowler was accused of helping to defeat Thomson, an opponent of the Equal Rights Amendment an supporting Gary Myers, a pro-ERA Republican who defeated Thomson.

Fowler had claimed that the federal court had jurisdiction over her case because her expulsion was done by the state through the Alexandria committee. But Lewis said in a five-page opinion that the Alexandria committee "was not acting as an arm of the state or under color of state law when they charged and found her guilty of violating" its by laws. He said the committee was acting as a party organized under the Virginia Democratic Party plan that the judge said is "a private entity."