The Democratic Party, capitalizing on an improved direct-mail fund-raising operation, has significantly reduced the Republican Party's overwhelming cash advantage.

Despite the gains, nationally based Republican committees raised $246.1 million in the 1983-84 election cycle to $58.8 million by Democrats. In 1981-82, the GOP raised $191.1 million to $31.7 million raised by Democrats.

The figures suggest the Democrats have begun to reverse what had been a growing GOP advantage. In 1977-78 the GOP raised 3.2 times more than Democrats and in 1979-80, 4.5 times more.

Roger Craver of Craver Mathews Smith & Co., which does direct-mail campaigns for Democratic national, congressional and senatorial committees, said Democratic cash from direct mail grew from $2.5 million in the 1980 election cycle to$17 million in the last election, nearly a sevenfold increase.

Phil Smith, finance director of the Republican National Committee, said that Republican direct-mail fund-raising during the same period doubled to $60 million.

The most significant gains for the Democrats have been in the amounts of money channeled by the major party committees into House and Senate contests. From 1981-82 to 1983-84, this grew from$3.3 million to $6.4 million.

The amount put into House and Senate contests by major Republican committees decreased from $18.7 million to $17 million.

These figures understate the Republican advantage. The GOP spends much more than the Democrats on polls, computerized analysis of district and precinct voting histories and demographic studies.

This information is made available to candidates, often at little or no cost. In addition, the GOP subsidizes state parties far more than does the Democratic Party.

The Federal Election Commission yesterday released figures showing that Democratic committees raised $71.1 million in 1983-84. But a Democratic National Committee spokesman said this includes about $12.9 million in loans, later repaid, for a 1983 telethon.

"It is money in, money out" and should not be counted when making fund-raising comparisons, the spokesman said.