If Ronald Reagan sticks to the letter of what he is now saying, Sen. Howard Baker is eliminated as his running mate on the narrow grounds of the abortion issue alone -- a decision of profound importance affecting much more than the vice presidency.

In a recent conversation aboard his campaign plane, Reagan was not at all evasive. He told us he "must pick someone who is seen as supportive" of him on key issues. That would not bar "minor disagreements." But Reagan does not consider abortion minor. And on that question, Baker's record is rated extremely good by the pro-abortion lobby and extremely bad by the anti-abortion lobby -- exactly the opposite of Reagan's. If Reagan means what he says, eliminate Howard Baker.

There is no evidence so far of serious discussions about vice-presidential selection between Reagan and his closest aides, whose preference for Baker is well known though unofficial. They realize a Reagan-Baker ticket would give Reagan instant respectability where he needs it most: The Republican National Committee, the U.S. Senate, corporate board rooms and the Washington press corps.

Baker is the one obvious vice-presidential possibility unacceptable to the right -- particulary the New Right, the populistic conservative movement that is linked with fundamentalist Christianity. But nobody really believes rank-and-file conservatives would desert Reagan if he selected Baker. Thus, rejecting Baker on ideological grounds not only would be viewed by many Republicans as an act of intransigent folly that portends the worst for a Reagan administration; it also might blight his chances for election.

Actually, Reagan's concern about Baker is not electoral slippage on the right but President Baker's performance as his possible successor. Whether or not this 69-year-old candidate has intimations of his own mortality, seldom has a prospective nominee insisted so often and so compellingly that his anointed successor strictly follow the course of the departed leader.

Although Baker's support of the Panama Canal treaties most outrages the right, that is the irrevocable past in Reagan's view. Contrary to published reports, Baker follows Reagan in backing Kemp-Roth tax reduction. He also shares Reagan's opposition to SALT II.

But on abortion, Baker does not merely fall into the right-to-life rogues gallery for opposing the anti-abortion constitutional amendment. He has been consistently pro-abortion on Senate roll calls over federal funding for abortions. Lifeletter, an anti-abortion newsletter, attributes 27 pro-abortion votes and only one significant anti-abortion vote to Baker. The National Abortion Rights League gives him a No. 1 -- its best pro-abortion rating.

On Feb. 15, Reagan sent a telegram to the National Pro-Life Political Action Committee promising to pick only a "pro-life running mate." More than two months later in our airborne interview, we asked whether he stood by that obscure promise. This dialogue followed:

A. I said that in the context of my belief and particularly in view of today's circumstances . . . I think I must pick someone who is seen as supportive of the things that I have been telling the people that I would do. In other words, if it fell to his lot to follow through, he would continue with the programs and things I have advocated.

Q. Across the Board?

A. Yes [except] maybe some minor disagreements someplace.

Q. You would not consider that [abortion] minor, though?

A. No, I wouldn't.

Shortly thereafter, Reagan confirmed in a Wall Street Journal interview that "it would be hard for me to accept" a pro-abortion vice president "because we're talking about a human life." He tended to downgrade the Panama Canal as a vice-presidential test: "I don't know how much the past would influence me."

The only escape hatch from all this is Baker's assertions that he personally opposes abortion and votes for federal abortions only where "the health of the mother is endangered" -- a classic bailout that neither Reagan nor the "pro-life" lobby accepts. Reagan now says he erred as governor of California in signing a "health-of-the-mother" law that led to wholesale abortions in the state.

Other vice-presidential possibilities are acceptable to the anti-abortion lobby -- Sen. Richard Lugar and Rep. Jack Kemp definitely, George Bush marginally. It is the respected Senate minority leader alone who poses the dilemma for Reagan: reject him for his stand on abortion, and be branded as a zealot; name him as running mate, and be condemned as a politician whose pledges can be ignored.