While Ronald Reagan flew through the Southwest attacking President Carter this week, his aides were focusing on the more immediate tactical problem of trying to ease George Bush out of the Republican presidential campaign.

Behind a facade of unconcern about anything that Bush says or does, Reagan strategists are worried that Bush's continued presence in the race is making it increasingly difficult for Reagan to demonstrate that he has the broad base of support necessary for a successful campaign against the president.

The concern in the Reagan camp is not that Bush will suddenly catch fire and go on to win the nomination. Everyone from the candidate on down is convinced that Reagan's lead is too large for that.

Instead, the fear is that Bush's mounting criticisms of Reagan -- labeled "strident" by one Reagan aide -- will open old party wounds and make it difficult for Reagan to bring Republican skeptics into the fold.

The numbers for Reagan are impressive, no matter which of the several private or public counts are used.

United Press International gives Reagan 592 delegates, while Reagan's own count and that of several publications put him in the mid-600s -- with 998 delegates needed for nomination. A total of 251 delegates will be chosen during the next three days and Reagan is expected to win at least two-thirds of them. This would enable him to clinch the nomination June 3 in his home state of California, where he now leads Bush in one respected public poll by 7 to 1.

But no one in the Reagan camp wants to wait until June 3.

The Reagan campaign has less than $1 million remaining of the $17.6 million it is allowed to spend in the primary campaigns under federal law. As long as Bush remains a candidate, a Reagan aide pointed out today, some of this money must be kept for contingency purposes.

Bush said Thursday in Austin that he would stay in the race even if he were beaten 90 to 1 in Saturday's Texas primary. If he holds to this pledge, the Republican National Committee will have to remain neutral and will be unable to spend any of its funds on behalf of Republican presidential candidates.

The Reagan hope is that if Bush gets out, the GOP National Committee could then become active on behalf of the only remaining Republican presidential candidate.

But Reagan and his aides seem convinced that Bush means what he says and that there is no way to force him out. Accordingly, the Reagan campaign is operating on the premise that a Republican senator once suggested the United States should pursue in Vietnam -- declare the war won and call the troops home.

Reagan chief strategist Richard B. Wirthlin predicted two weeks ago that it would be apparent on May 6, the date of the Indiana, North Carolina and Tennesse primaries, that Reagan, to all intents and purposes, had clinched the GOP nomination. If this forecast comes true, Reagan will spend much of the next few weeks at home and his campaigning on the road will shift to a general electionstyle format.

While estimates vary, the view in the Reagan camp now is that the former California governor will win 65 of the 80 Texas delegates, including eight that will be awarded to the primary winner later this month. He is expected to win at least 17 of the 32 Tennessee delegates, and 24 of the 40 North Carolina delegates. Reagan is believed likely to sweep the 54 delegates in Indiana and win most of 31 delegates that will be chosen in a Colorado state convention Monday. He does not count on any of the 14 District of Columbia delegates, who will be chosen next Tuesday.

For his part, Reagan has been acting as if Bush does not exist, never mentioning his name unless he is asked about Bush. But Reagan acknowledged at a press conference here today that he is disturbed by some of Bush's recent criticisms, especially his statement that Reagan's proposed federal income tax reduction would increase inflation by 30 percent.

And Reagan took one shot at former Texas congressman Bush in the process of dismissing a question about the outcome of the Texas primary.

"He's done rather well in his other home states," said Reagan in a reference to Bush's victories in Massachusetts, Connecticut and Pennsylvania. "I don't have that many home states myself. I'm running as a Californian."