Acting with rare dispatch, the Democratic Party executive committee recommended a series of modifications yesterday in the process for picking delegates to the 1980 presidential convention.

National Chairman John C. White said he hoped the 10-year process of rewriting party rules "is now near its end," but some further revision is inevitable when the full Democratic National Committee meets here in June.

Donald Fowler, the South Carolina chairman, conceded after yesterday's meeting that the major new rule recommended by the executive committee yesterday is flawed and has to be revised.

Fowler's proposal, which had White's support, would make it somewhat harder for minor candidates to win delegates to anyone getting 15 percent of the votes in a primary or causus and let states reduce that percentage if they wished.

A proposal advanced by a party rules revision commission earlier this year - and criticized as an effort to impede any challenge to President Carter's renomination - would have set the floor at 15 percent and raised it to 25 percent by the final month of the primary and convention season.

Fowler's proposal set the "cutoff" at 15 to 20 percent for all caucuses and conventions and at least that high for most primaries.

But in presenting it, Fowler inadvertently eliminated language preventing a plurality winner from gainning all the delegates at stake in a particular district, so the provision will require further refinement.

As expected, the executive committee approved a White compromise to set the filling deadline for 1980 primaries anywhere from 30 to 90 days before the primary date, and to confine most of the delegate selection to a 13-week period in the spring of 1980. Exceptions would be allowed for states like lowa and New Hampshire which already have winter caucuses or primaries.

The executive committee also dorsed a suggestion that the convention size be increased 10 percent to give voting rights to Democratic governors, members of Congress and senior party officials.