Fresh from an overwhelming victory Oct. 15 in the submissive "electoral college" that named him Brazil's next president, Gen. Joao Baptista Figueiredo has decided to risk his newly won prestige in an effort to preserve the government's majority in elections here.

On Wednesday, 46 million Brazilians will go to the polls to choose senators and representatives to the next National Congress and state assemblies. It is the first time since the military seized power in a coup here in 1064 that a popular congressional election has immediately followed the presidential selection process, which is carefully controlled by the military and involves no popular participation.

The juxtaposition of the congressional election with the so-called "indirect" election that picked Figueiredo as successor to Gen. Ernesto Geisel has led government and opposition to look upon this week's balloting as an unofficial referendum on the choice of Figueiredo. In the words of opposition party leader Sen. Paulo Brossard, the congressional election will be "a plebiscite on all these years of arbitrary rule."

From Figueiredo's point of view, the initial indications are non too promising. Polls and unofficial soundings taken across the country indicate that the Brazilian Democratic Movement, the nation's only legal opposition party, will make significant gains at the expense of the official government party, and may even win a narrow majority in the lower house of Congress.

The government is assured of a majority in the upper house as a result of an official decree in April 1977 that took one-third of the seats in the upper house away from popular voting and gave the president the power to fill them by appointment.

The poll results are particularly one-sided in major urban areas, where demonstrations against the rising cost of living and political repression have become commonplace in recent months. In the state of Sao Paulo, home to more than 20 percent of the nation's electorate, polls show the opposition leading the government forces by about a 2-to-1 margin.

In years past, such figures would have been scant cause for concern by Brazil's military rulers. Congress here has been virtually powerless since the coup of 1964 and presidents have not hesitated to invoke their arbitrary powers to shut it down at te first signs of opposition clamor.

But as a result of reforms due to go into effect on Jan. 1, The New Congress will gain a significant degree of latitude and independence. When he takes office March 15, Figueiredo, unlike his predecessors, will not be able to close Congress or deprive individual members of their mandates when they show signs of resisting his will.

The prospect of increased opposition strength in this less-fettered Congress led Figueiredo earlier this year to predict a backlash among hard-line elements within the military. If the Democratic Movement were to win, he said, "the situation will explode. Either I explode along with it, or I go along with them and we head into a regime far worse than this one."

Figueiredo has since backed off from that statement, explaining that what he really meant to say was that it would be easier tor him to govern with a majority. Political observers here however, are unanimous in predicting that Figueiredo's political inexperience and hot temper will make for a period of political turbulence if opposition forces score the gains that are expected.

Hoping to stave off this unwelcome possibility, Figueiredo has plunged into a whirlwind schedule of campaign appearances all over the country. And in a move that has made the opposition leadership openly gleeful, the government party has geared its entire campaign strategy to pledges of unwavering support of Figueiredo and, whatever actions he takes as president.

In Sao Paulo, for example the freeways are lined with giant billboards and the walls plastered with posters showing Figueiredo's smiling face and the caption "ARENA" - the initials the government party is known by - underneath it. "Mr. President, Sao Paulo supports you because Brazil is in a hurry," the accompanying slogan says.

ARENA campaign workers are also making use of "The Book of the Thought of Gen. Figueiredo," a glossy compilation of statesmanlike pronouncements on major political and economic issues. But the impact of "The Little Green Book," as it is popularly known, has been diminished by jokes and widespread press comment triggered by the gaffes and non sequiturs that continue to be Figueiredo's trademark.

"I intend to open this country up to democracy," he told reporters minutes after being informed of his election. "And anyone who is against that, I will jail, I will crush."

The opposition, however, has been prevented from exploiting fully what they see as figueiredo's vulnerability by regulations that seriously restrict campaign activities. A law passed after the Democratic Movement scored significant gains in the 1974 elections prohibits political advertising or debates between candidates on television or radio.

In Rio de Janeiro recently, an opposition senator seeking reelection was prevented from conducting a campaign march down a major downtown street by an estimated 1,000 military policemen armed with guns, nightsticks and trained attack dogs. In Bahia, the wife of the opposition candiate for senator was arrested while pasting up posters for her husband.

"The 1974 elections are the closest thing we've ever had to genuine free elections under this regime, and the government was beaten badly," said a Rio newspaper editor. "They've obviously decided to do everything they can to prevent that from happening again."