TO POLITICAL PARTISANS, vote-changing is seldom judged in the abstract; what matters is which way the changes go. To some extent, we look that way at the current maneuverings in Richmond over the federal Equal Rights Amendment. We could wax very indignant about how Republican Sen. A. Joe Canada Jr. of Virginia Beach, who wants to be lieutenant governor, torpedoed the ERA in the state Senate Thursday by suddenly withdrawing his support. And then we could praise Rosalynn Carter and other pro-ERA lobbyists to the skies for trying to switch a vote or two the other way so that ratification can be reconsidered and approved.

We do think the anti-sex discrimination amendment is reasonable and just, and should be ratified. It is encouraging that its persistent advocates, ably led by Sen. Clive L. DuVal II and Del. Dorothy S. McDiarmid of Fairfax County, have been picking up support in the General Assembly. Yet there is also something else at stake in Richmond: the legislature's obligation to debate and vote squarely on a proposed amendment to the federal Constitution, which has been submitted by Congress to the states.

For four years ERA's opponents, such as Del. James M. Thomas of Alexandria, have frustrated every attempt to gain a straightforward record vote in either house. This year that logjam was finally broken when a majority of the Senate agreed to bring the question to the floor. If Mr. Canada had not changed his mind, the ratification measure would have been approved and sent on to the House, where an up-or-down vote might finally have been obtained. That is the strategy that Mrs. Carter and others are now trying to revive.

The outcome in Richmond is probably not crucial to the ERA's prospects for ratification by 1979. With Indiana's favorable vote this month, the amendment has been endorsed by 35 of the required 38 states - though two have tried to rescind their approval. Important tests are coming up in several legislatures, such as North Carolina and Nevada, where the outlook is fairly promising. It would be a happy surprise if Virginia did join the pro-ERA ranks. Especially in an election year, House members as well as senators have an obligation to let the voters know where they stand on ERA.