JAMES T. MCINTYRE, an unassuming, hardworking 37-year-old lawyer who has been minding the store at the Office of Management and Budget during the Bert Lance troubles - and, in fact, according to some, since well before those troubles set in - has now been nominated by Mr. Carter to be director of OMB. He will preside over a $500-billion budget and a government agency that is, above all others, the keeper of the Carter political flame: The President has staked his credibility - in a way, his very presidency - on pledges that the OMB is essential to fulfilling. Is Mr. McIntyre the man for the job? The anwer depends on how Mr. Carter sees the job.

A comparison of Mr. McIntyre's qualifications with those of his hapless predecessor is instructive. Mr. McIntyre, by virtue of his 10 months or so of toil at OMB, comes to the directorship with a much greater familiary than Bert lance had at the beginning (or the end) with the complexities of the federal budget and the obduracies of the federal establishment. He has a better understanding of the intricate workings of both the budget and management aspects of the job. That is one key difference. Another is that Mr. McIntyre, unlike Mr. Lance, is neither an intimate friend of Mr. Carter nor an A-level political agent and adviser. No one envisages Mr. McIntyre's assuming a Lance-like role as troubleshooter, ambassador to business, counsel-giver, rebate-squasher, economic gurn and the rest. Mr. McIntyre will evidently be a relatively narrowly focused, working director of OMB.

When we say that the success or failure of the Carter presidency depends in considerable measure on what happens at OMB, we have in mind both the general importance of the agency and its particular relevance to some of the President's campaign pledges and regularly restated objectives. The budget-making process is the setting in which the priorities are set, the trade-offs made, the decisions reached about what the President wants to do and how it will be financed and how the burdens will be shared. It is, in other words, the political heart of any administration. And it is also the federal agency that makes the government's principal impact on the economy of the nation as a whole. These things are always true. What is distinctive in the Carter administration is the fact that the President has made a variety of promises concerning a balanced budget, a reorganized government and certain vastly expanded programs that must be achieved (if they can be achieved at all) in the workings of Mr. McIntyre's agency.

When you put these things together - Mr. McIntyre's particular qualifications, his instinct for the "nitty-gritty," and the enormous political and economic stakes riding on the success of his enterprise - the possibility immediately suggests itself that Mr. Carter has something special in mind: becoming, in a secse, his own director of OMB. The President who this fall is said to have got into the budget skirmishing among the agencies and between the agencies and OMB at a much earlier stage than Presidents have previously done, seemed determined to direct his own budget operation on the cosmic, politically and economically fraught issues. He had what you might even call a kind of tryout with Mr. McIntyre, and evidently is satisfied with the result.

If this is the case, and if Mr. Carter does attempt to be the de facto chief budget officer of his own administration, we expect that he will only be inviting more trouble - squandered time and lost opportunities to engage in the larger policy and political roles that only a President can fulfill. It is possible that Mr. Carter's nuts-and-bolts involvement in the coming budget only signified a concern for the quality of his administration's first full budget. We hope so. And we hope that his nomination of Mr. McIntyre is based on confidence that Mr. McIntyre is the man for the job - not, dismaying thought, that he will only remain in fact the deputy while Mr. Carter himself takes over the office.