A Democratic Party rules commission voted yesterday to shorten the primary election season in 1980, but it left a loophole that probably would allow New Hampshire to hold its early, first-in-the-nation primary at least one more time.

The Party's Commission on Presidential Nomination and Party Structure, headed by Michigan Democratic Chairman Morley Winograd, voted 41 to 12 to limit the nominating season to three months, cutting the period almost in half from that of 1976.

No delegate selection caucuses, conventions or primaries could be held before the second Tuesday in March or after the second Tuesday in June under the new rule.

New Hampshire's primary was held in late February in 1976, and state party leaders appealed Friday to President Carter - who won that contest - to help them preserve their traditional kickoff date.

With the support of presidential aide Mark Siegel, a Winograd Commission member, the commission voted a loophole in its new rule which members said should take care of New Hampshire at least in 1980.

It says, in effect, that any state party which can show it has taken "provable, positive steps" to carry out the party mandate can have the status quo preserved if those efforts fail.

In New Hampshire, where Republicans control both the governorship and the legislature, any effort to move back the primary is foredoomed.

On the other hand, Democrats control the legislature and the governorship in Massachusetts, which also held its primary earlier than the new rules allow, so Massachusetts will presumably be forced to shift its date back.

Commission aides said half a dozen states held delegate-selection caucuses earlier in 1976 than the new rules would allow, and those caucuses - set by party rules - would have to be shifted to comply.

The new rule would also forbid any state that now selects its delegates within the prescribed three-month period from moving to an earlier or later date outside that period.

In a separate vote, the commission said there should be no exemption in 1980 for any state holding a primary which allows Republicans to vote in the Democratic contest. Wisconsin, Michigan and Montana were granted such an exemption from the "closed primary" rule in 1976, but would not be allowed the loophole in 1980.

The commission met late last night and is to meet again today to complete its recommendations, which are subject to approval by the Democratic National Committee.