Democrats moved today to require that half the delegates to the party's 1980 national convention be women.

The action, following almost a decade of lobbying by women's groups, would dramatically alter the makeup of the party's nominating convention, traditionally dominated by men.

At the Democrats' 1976 convention, 34 percent of the 3,008 delegates, or about one-third, were women. In 1972, the year the party nominated Sen. George McGovern (D-S.D.) for the presidency, the figure was 38 percent.

Although the role of women has been one of the party's hotly debated issues in the past, today's vote by the male-dominated party executive committee met with only token opposition.

The only voice raised against it was that of Henry Braden, a state senator from Louisiana, who said the rule change "smacks of quotas."

"If you're going to play quotas in this respect, you might as well open it up to all quotas," whether they have to do with race or anything else, Braden said.

Ruth Harvey Charity, an executive committee member from Virginia, said women deserved to have the representation at the convention because "it's usually the women who are out at the polls," and "who are on the phone working" for the party.

The move came as party leaders, gathered here for the Democratic midterm conference, approved a preliminary call for the 1980 convention. A committee to decide the site of the convention is to be appointed Friday.

In addition to requiring that half those attending be women, the executive committee also made the convention considerably larger than in previous years, with 3,313 delegates and 2,042 alternates.

Under the formula approved today, the District of Columbia will have 19 delegates and 19 alternates. Maryland will have 59 delegates and 40 alternates. Virginia will have 64 delegates and 42 alternates.

The sudden capitulation on the equal-division issue caught feminist leaders by surprise. "This is incredible and I'm delighted," said Mildred Jeffrey, head of the National Women's Political Caucus and veteran Michigan Democratic leader.

The question of equality has been around since Democratic reform efforts began in 1968. On the eve of his nomination in 1976, President Carter was confronted by the Democratic Women's Caucus with a demand that he support a requirement that 50 percent of the 1980 delegates be women. He finessed the issue then with a generally worded promise.

But party officials said it was clear that the issue would come up again in 1980, and they decided to dispose of it now rather then let it remain as a potential problem for the president.

Some of them acknowledged that there might be litigation challenging the new rule, but pointed out that the party's charter and internal party decisions exempt the equal-division clause from the general ban on quotas for demographic groups.

In another action, the executive committee voted to close one more loophole some states might use to avoid choosing convention delegates by the proportional representation method.

They outlawed the creation of small, special districts to elect single delegates to the convention -- a method Alabama used in 1976 and which other states were reportedly contemplating adopting in 1980.

The effect of this change and earlier rules amendments is to require virtually all delegates to the 1980 convention to be chosen to reflect the proportion of the popular vote each presidential candidate receives in a district.