CHESAPEAKE, VA., SEPT. 15 -- An ebullient Marion G. (Pat) Robertson, saying that God and the American people have urged him to seek the White House, declared today that he will formally enter the race for the 1988 Republican presidential nomination.

"I feel like somebody strapped on a couple of jet engines on my back and we're ready to take off," the jaunty Robertson told a shouting, foot-stomping group of volunteers at a campaign office in this Virginia Beach suburb. He said he would make an official declaration of candidacy on Oct. 1.

Robertson said he felt confident because of his victory in a GOP straw poll in Iowa last weekend and because he had met and surpassed the ambitious goal he set for himself a year ago when he launched the exploratory phase of his candidacy.

The 57-year-old religious broadcaster said last September he would run if he could obtain signatures from 3 million registered voters asking him to do so. Today, standing beside a wall of petitions stacked four feet high, he said he had "accumulated the names of 3.3 million Americans to encourage my candidacy."

Robertson's campaign manager, Marc Nuttle, said the candidate's volunteers would continue gathering petition signatures as an organizing tool. Robertson said he hoped to get 7 million signatures by the start of the 1988 primaries. "It only takes 7 million people to win {the GOP nomination}," he explained.

Robertson is a charismatic evangelical minister who founded and chairs the Christian Broadcasting Network (CBN). He has told campaign audiences that he has a direct call from God to seek the presidency.

Today, when a reporter asked if God is urging him to run, Robertson referred to the overflowing pile of petitions around him and said, "The people are urging me to run." He added that he believes no one should seek the presidency without a conviction that "it is indeed the calling of God."

When he began his presidential pursuit a year ago, Robertson indicated that moral concerns were the major issues motivating his candidacy. "What we are facing is not a governmental problem, it is a moral problem," he said then.

Today he did not discuss specific moral issues, saying only that "there is an agenda that I believe will be very helpful" in that area. Instead, he emphasized education and foreign policy questions in discussing the substance of his platform.

He was critical of the U.S.-Soviet arms control agreement that the Reagan administration is moving to implement. He said he would campaign against the proposed treaty "as it now stands."

Robertson's years as a evangelical minister on television have won him a fervent following in the Christian community, but he also has unusually high negative ratings in opinion surveys among people who are suspicious of his religious practices. He said today that he is confident he can reverse the negatives by emphasizing his education and his business success in broadcasting.

"It takes a little while on TV, four or five minutes," he said, "to switch 40 percent of the totally negative people into 'let's take a second look.'"

Robertson has been testing a series of television advertisements in South Carolina. The ads, three 30-second commercials shown 800 times last week in two markets, present him as a businessman who is informed on a broad range of policy questions, and suggest voters "get the full picture of a man uniquely qualified to be president."

The candidate released a new campaign biography today that corrects some of his assertions in earlier official biographies. The biography no longer says that Robertson served on the board of directors of United Virginia Bank or that he attended graduate school in London. Referring to his service in the Marines, the biography says that he served as an adjutant, or clerk, in a unit that was in combat; it does not say Robertson himself was a "combat officer," as he had previously written.