Separated by just six city blocks across the teeming heart of downtown Denver, a pair of lively national conventions this weekend provides a case study in the changing fashions of American politics.

In a fancy new hotel on Denver's 17th Street, the Wall Street and Madison Avenue of the Rocky Mountain West, the 13th convention of the National Right to Life Committee is alive with political activity, with officeholders speaking virtually from dawn to dusk and with competing presidential contenders striving vigorously for support from the antiabortion community.

In an older, less posh hostelry near Colorado's capitol, the 20th convention of the National Organization for Women has just one elected official speaking at its numerous forums, and the only sign of presidential politics is a stack of Geraldine Ferraro bumper stickers, vintage 1984, on sale for the discount price of 25 cents apiece.

Both groups agree it was sheer coincidence that led them to schedule their conventions on the same weekend in the same town. But there's nothing happenstance about the organizations' conflicting attitude toward electoral politics.

"We invited people in a position to help us," said Right to Life Committee President J.C. Willkie, explaining why so many political figures were on the convention program.

"We've made a deliberate choice to move away from politicians of either party," said NOW President Eleanor Smeal. "We've learned that it doesn't help to be dependent on the whims of political figures."

It was not always thus, on either side.

When the Right to Life group organized shortly after the 1973 Supreme Court decision holding that states could not completely outlaw abortion, it was a splinter organization far removed from the political mainstream. It was not until the 1980 election, when most antiabortion organizations united behind Ronald Reagan's campaign for the presidency, that the antiabortion stand became a significant factor in national politics.

This weekend's convention, though, could have passed for a Republican Party rally. The first major speaker was Reagan, appearing over a special television hook-up from Washington. Three potential contenders for the GOP's 1988 presidential nomination -- Sen. Robert J. Dole (Kan.), Rep. Jack Kemp (N.Y.) and the Rev. Marion G. (Pat) Robertson -- were to be here to shake hands and come out speaking at the convention's general sessions.

Another GOP possible hopeful, Vice President Bush, was also invited to speak, but ran into a schedule conflict. The vice president's political action committee, however, did buy a full-page ad in the convention program.

If anything, the political activity at the Right to Life meeting this weekend was reminiscent of yesteryears' gatherings of NOW.

"We used to have conventions packed with presidential candidates," Smeal said. "In '76 they were all over the place. In '83 we probably had half a dozen of the Democratic candidates giving speeches."

And in 1984, after NOW had endorsed Walter F. Mondale for president, the convention delegates invited Mondale to speak and then virtually demanded that he choose a woman as his running mate.

But Smeal said the 1984 experience had prompted her group to move away from the major political parties. "We embraced the Democrats, but the Democrats never put women's issues high enough on their burner," she said. "And now they have the gall to blame that disaster in 1984 on the women's movement, on the 'special interests.' "

Except for some minor heckling, which led to the arrest of a small number of antiabortion demonstrators, the two groups meeting here have generally left one another alone.

Organizations taking part in the NOW convention range from the "Young Socialist Alliance" to the "Evangelical Women's Caucus," and the seminar sessions range widely over topics that included automobile insurance, child care and gay rights.

Smeal said a major goal of the convention is to expand the group's focus to include international issues of feminist concern as well as the level of federal spending on military items. "We'll never get our social programs approved as long as all that money is pouring into defense," she said.

The Right to Life meeting's agenda focuses almost entirely on abortion. But the seminar schedule is full of sessions on political topics: "Political Caucuses," "Electing Pro-Life Candidates," "State Legislative Lobbying."

The oratory is peppered with denunciations of the Supreme Court and its rulings that the individual's right to privacy prevents government from banning abortions.

Robertson called the high court's justices "despots" and observed, "You can search the Constitution throughout all of its pages . . . and you will never see the term 'right to privacy.' "

Kemp called the 1973 decision a "tragic betrayal" of American values and said that only those "who respect the sanctity of life" should be appointed to federal judgeships.

Dole is scheduled to address the Right to Life meeting Saturday.