D.C. supporters of Sen. Edward M. Kennedy won a rule change last night that will send one more Kennedy delegate to the Democratic National Convention than originally calculated. However, the change may be contested by supporters of President Carter.

The D. C. Democratic State Committee approved the change in its delegate selection rules in a 29-20 vote that split largely along Kennedy Carter lines.

Under the committee's old rules, patterned after national party rules, Kennedy had been allocated 11 convention delegates when the District and Carter had been slated to receive eight. The rule adopted last night means that Kennedy will have 12 D.C. delegates and Carter will have seven. m

In the May 6 primary election, Kennedy won 62.5 percent of the vote to Carter's 39.5 percent. But the old rule, based on a complicated formula would have given Kennedy only around 58 percent of the District's delegates.

The new rule, based on a formula tied more closely to the popular vote, means that the delegation will more accurately reflect the vote percentages. But since the old rule was in compliance with national party rules, the delegation may be challenged by President Carter.