Senators running for reelection in 1988 are raising far more money in the early stages of the election cycle than they did in their previous races and are far more dependent on campaign contributions from political action committees (PACs), according to a study released yesterday by Common Cause.

The analysis by the self-styled citizens' lobby of information released last week by the Federal Election Commission reveals that the 32 senators who are scheduled to face the voters next year raised more than $20 million during the first six months of this year. Since they last ran in 1982, the 32 incumbents have raised nearly $41 million and have almost $28 million on hand.

Of the total raised during the first half of 1987, almost 35 percent, or $7.2 million, came from PACs, according to the study. That represents a significant increase from the senators' last campaigns, when, at a similar stage, PACs had contributed about $1.5 million, or 22 percent of the total.

Common Cause President Fred Wertheimer said the study illustrates the increasing dominance of PACs in congressional campaigns and underscores the need for campaign-finance overhaul legislation, which is pending in the Senate.

"It's clear we are headed into yet another national election where excessive campaign spending fueled by excessive PAC contributions will play a dominant role," Wertheimer said in a prepared statement. "It's absolutely essential for the 100th Congress to act to end the disgraceful way our congressional elections are financed."

Senate action on the campaign-finance measure has been blocked by a Republican-led filibuster since it reached the floor in early June. Senate Majority Leader Robert C. Byrd (D-W.Va.), who has made passage of the legislation a top priority, has scheduled another vote on ending the filibuster for early September, when Congress reconvenes.

Cosponsored by 44 Democrats and two Republicans, the legislation would create voluntary spending limits on congressional races, provide for public funding of general-election campaigns for those candidates who abide by the limits and restrict the participation of PACs. The spending limits would range from $950,000 in small states to a high of $5.5 million. PAC contributions would be limited to between $190,950 and $825,000 for Senate candidates.

The Senate's champion fund-raiser so far, according to the study, is Finance Committee Chairman Lloyd Bentsen (D-Tex.), who has received $5,084,044 since 1983, $3.8 million of it this year. Sen. Pete Wilson (R-Calif.) follows with $4.2 million.

Bentsen, whose committee is one of the most heavily lobbied on Capitol Hill, has also collected the highest amount from PAC contributions, $1.3 million, or about one-fourth of his total.

However, when it comes to PAC contributions as a percentage of funds raised, Bentsen trails many of his Senate colleagues. Six senators -- William V. Roth Jr. (R-Del.), John Melcher (D-Mont.), Quentin N. Burdick (D-N.D.), Malcolm Wallop (R-Wyo.), Chic Hecht (R-Nev.) and Byrd -- have received more than half of their total contributions for 1988 from PACs. Melcher tops that list, with 79 percent of the $513,039 he has collected coming from PACs.

At the opposite extreme is Sen. Lawton Chiles (D-Fla.), who has raised $754,774 despite a pledge to take no contributions from PACs, out-of-state contributors or in excess of $100. Though Chiles' self-imposed restrictions would appear to invite opposition, he faces no challenge thus far.