DENVER, SEPT. 28 -- On a crisp autumn afternoon, a somber Rep. Patricia Schroeder (D-Colo.) came to an outdoor amphitheater in a city park ringed with golden aspen trees to announce a personal decision that clearly caused her pain: She will not run for president in 1988.

Schroeder told a sympathetic crowd that three months of touring the country as an "exploratory" candidate taught her two lessons: First, it is already "too late to get in and . . . catch up" with other Democrats; second, the presidential nominating system is "so isolating" that she could not wage a campaign suited to her highly personal political style.

"I could not figure out how to run," she said, stopping to wipe away tears, "how to run and not be separated from those I serve . . . . I could not bear to turn every human contact into a photo opportunity."

Although Schroeder looked and talked like a candidate all summer, and received warm receptions around the country, she began having second thoughts three weeks ago as to whether she could win and whether she could wage her own style of campaign.

On Labor Day weekend, as Schroeder and her family ended a Mediterranean cruise and boarded a flight back to the United States, she found a copy of USA Today in the Milan Airport and flipped to the political news.

Suddenly, Schroeder's husband recalls, her family heard her groaning in anger. "Oh my God!" she said resentfully. "I'm supposed to be in Florida tomorrow for one of those huge rallies."

Faced with the prospect of months of "huge rallies" and other campaign activities, she concluded that she would not be able to run "the kind of campaign where you can stay in touch with the people."

The 47-year-old lawyer, now serving her eighth term in Congress, left the strong impression that she would try to find a way to run her kind of campaign for president in 1992 or beyond. "I'm going to keep figuring on this . . . and we're going to learn how to use the skills that we learned this summer."

But Schroeder also said she is not yet convinced that American voters are ready to make a woman president. She said women's status in politics has improved dramatically in recent years, but "we're not completely there yet."

Irene Natividad, who heads the National Women's Political Caucus, praised Schroeder's decision, calling it wise and adding that neither Schroeder nor the caucus believes in a symbolic candidacy. "Sending a message is not enough," she said. "We're in it to win." She added that the same factors that made former senator Paul Laxalt (R-Nev.) and Sens. Sam Nunn (D-Ga.) and Bill Bradley (D-N.J.) opt out of the 1988 campaign worked on Schroeder.

Schroeder brushed aside questions about the possibility that she might become a vice-presidential candidate in 1988: "Are you kidding?" she said. But she did not completely close that door.

Schroeder's withdrawal from a campaign she never quite entered was clearly a painful moment for her. At the start of her speech, she was direct and chipper. But she broke down just after she said "I will not be a candidate." The rest of her remarks were punctuated with sobs. Her family stood on the platform with her, looking glum.

Schroeder's comments about the presidential nominating system had a sadder-but-wiser quality. She showed no bitterness and gave a glowing account of her three months on the road this summer as an almost-candidate for president.

Although few analysts predicted the Schroeder decision would have a major impact on the Democratic field, the most immediate beneficiary may be Sen. Paul Simon (D-Ill.).

The Washington Post-ABC News Poll showed Simon dropping from 13 percent in the June poll, in which Schroeder's name was not included, to 5 percent in the September ranking, completed last week, which included Schroeder. No other candidate's standing was significantly affected by Schroeder's presence on the questionnaire. A question asking for second choices indicates that Simon could regain about 3 points of his 8-point loss with Schroeder out of the picture.

In a sense, the exploratory campaign known as "Schroeder in '88?" began in 1972 with the last woman to seek the presidential nomination of either major party -- former House member Shirley Chisholm (D-N.Y.). Schroeder has said that Chisholm's example helped prompt her to give up her Denver law practice and run for Congress in 1972. She won that race and has never had a close challenge since in this strongly Democratic city.

Until May of this year, Schroeder was supporting the 1988 candidacy of her fellow Coloradan, Gary Hart. When Hart dropped out in May, Schroeder began examining the possibility of running herself. Liberal Democrats, and particularly active feminists, began suggesting that she seek the nomination.

Schroeder spent most of the summer on the road, trying out her message and soliciting contributions. Determined not to be just a "women's candidate," she emphasized her experience in military affairs and foreign policy. She had a no-holds-barred campaign style, swinging away at President Reagan with great elan.

"Ronald Reagan's approach to defense spending is pure Imelda Marcos," she said. "If the shoe fits, charge it."

Personally thrifty, she said initially she would not run unless she could raise $2 million by this month. It appears that she took in about $1 million. She could legally keep those contributions for other political candidates or campaigns of her own. She could also return the money to the donors.

In the end, it was not money but rather the method of running for president that convinced Schroeder not to make the race this time. "I keep asking," she said in a Washington Post interview two weeks ago, "can somebody give me a model of a nontraditional . . . campaign?" Today, she said she had not yet found the model but would keep looking.

In addition, Schroeder and her husband, James Schroeder, a Washington lawyer who has been her de facto campaign manager, began recently to take a close look at the various states' rules for picking delegates to the Democratic National Convention. The research showed, she said, that candidates must enter the race earlier than she "to deal with all the delegate selection and all the other things . . . in the states."