Challenges for Brazil's new leader. Sarney faces early political and economic tests as he settles in

Brazil's new president has begun his job in earnest. But the nation is watching to see whether he can fill the shoes of the man he was to serve as vice-president and for whom he was a stand-in for nearly six weeks. The task will not be easy for Jos'e Sarney, the poet President from one of Brazil's minor states. He assumes the mantle of the late President-elect Tancredo de Almeida Neves, who in less than a year had become a figure respected by millions of Brazil's dispossessed. The nation has yet to recover from the loss of Dr. Neves.

Meanwhile, Brazil is mired in economic difficulties, aggravated by recession, and beset by social tensions arising out of unfullfilled hopes.

But another military takeover that would annul Brazil's return to democracy appears out of the question, nor is it likely that the reform movement, which Neves christened ``New Republic'' will be hastily buried. What is at issue is who will supply effective leadership of the movement and whether power struggles and ``politics as usual'' will deflect the course of reform.

So far, Mr. Sarney has had no real opportunity to show he has the necessary stature for the job. But there is no challenge to the legitimacy of his role. Legally he has a six-year mandate. A new constitution, to be drafted beginning late next year, will probably cut the presidential term to four years. Neves accepted this in advance, morally binding Sarney to follow suit.

The Neves legacy, in fact, can be a powerful instrument in Sarney's hands. The entire Neves episode has brought Brazil a unity unique this century. This should afford Sarney the opportunity to bring about badly needed and long-overdue changes in the social sphere such as land reform, unemployment insurance, and a broadening of the voting franchise.

The opposition is small. In the National Congress, the coalition that backed Neves and Sarney in last January's elections has a safe majority of nearly two thirds.

Nevertheless, the future is not without perils.

In the first place, the Neves legacy is a two-edged sword. Sarney is pledged to follow the directions mapped out by Neves -- directions that appear somewhat conflicting since they encompass both government frugality and social-oriented measures plus the creation of job opportunities. But Sarney must avoid allowing the inheritance to tie his hands, lest others challenge his authority.

Sooner or later Sarney must begin to govern in his own right. He will probably have a brief honeymoon. Then there is the Cabinet. For the time being, Sarney must stick with Neves's handpicked ministers. But eventually, he will want to appoint some of his own choices, people in whom he has personal trust and whose first loyalties are to him.

An opportunity for changes will come around mid-1986 when some members of the current team resign to become eligible to seek elective office.

Much will depend on the ability of the speaker of the Chamber of Deputies, Ulysses Guimaraes, to control his flock. In fact, Mr. Guimaraes could turn out to be the key leader in a government in which Congress rather than the executive will be the stronger branch.

In the end, some sort of partnership could shape up between Guimaraes and Sarney. Such an arrangement could resemble a modified and undeclared parliamentarianism, in which Guimaraes's role would approximate that of a prime minister.

The trend is already under way. During Neves' illness, Sarney put to congress the question of whether the governement should refloat a bank that collapsed. The matter is about to come to a vote.

And the same tactic reportedly will be used on the question of how to cover a budget deficit of some $10.5 billion, inherited from the preceding administration.

Despite the power-sharing prospect, Sarney should not be written off as inconsequential. Although he may lack certain traits that enabled Neves to lead the nation back to democracy under civilian governement, he is nonetheless a shrewd politician, dubbed by some Brazilian political analysts as an ``old fox.''