Virginia legislator Richard M. Bagley (D-Hampton), the powerful chairman of the House appropriations committee and a member of the General Assembly for 18 years, said today he will not seek reelection but will instead run for state Democratic Party chairman.

The decision was widely seen as a move by Bagley to shore up support for his expected run for governor in 1989. It also appeared to dim the hopes of some Democrats that Bagley would accept a draft this year for the party's nomination for lieutenant governor.

Bagley is one of 100 members of the House who would be up for reelection this fall.

Bagley, in December, dropped out of this year's gubernatorial race after spending about $300,000 and 18 months on the road in a futile effort to catch two better known Democratic candidates, Lt. Gov. Richard J. Davis of Portsmouth and state Attorney General Gerald L. Baliles of Richmond.

"My single reason for stepping down is the challenge of the party," Bagley said at a Richmond press conference. "But I would be less than honest if I didn't say I have high hopes for statewide office in 1989." Bagley, after dropping out in December, vowed to be "first in line in '89."

Bagley has been talking with party leaders, including Democratic Gov. Charles S. Robb, since the party's February Jefferson-Jackson fund-raiser in Richmond. He said today he has the support of the current party chairman, Del. Alan A. Diamonstein (D-Newport News), who has indicated he will not run for another four-year term at the party's state convention June 7 in Richmond.

"Alan and Dick are longtime friends," said Democratic Party issues adviser Bernard Craighead. "I think you can assume they talked about this." Diamonstein could not be reached for comment.

Bagley's decision comes just four days before a party deadline for candidates to file for statewide nominations.

Bagley, considered the most fiscally conservative Democratic candidate but a moderate on social issues, has refused to support or discourage a struggling effort by some conservative Democrats to draft him for lieutenant governor. He stuck by that decision today.

"I think that's dead," one party official said.

The lieutenant governor's spot, which has drawn five Republican candidates, has so far been left on the Democratic side to state Sen. L. Douglas Wilder (D-Richmond), a 15-year member of the Senate.

Some Democrats have expressed concerns, mostly in private, that Wilder, who is black, could spark a defection of white voters to the Republican Party candidates similar to that in last year's presidential campaign.

Wilder, as well as Bagley, Robb and other party leaders, has denounced those views and said candidates should be judged on the issues, not race.

Wilder, appearing at a forum with the Republican candidates in Richmond, said he is confident Bagley will not get into the campaign.

Meanwhile today, a group supporting a Baliles-Wilder ticket announced the names of several new supporters including state Sen. Robert C. Scott (D-Newport News), who is the only other black in the 40-member Senate.

Many in the state had expected Scott to support Davis. Neither Wilder nor Baliles has officially endorsed the idea of a Baliles-Wilder ticket. However they have not discouraged it either.

"This is a major coup," said 3rd Congressional District Chairman Lawrence Framme, a Richmond attorney and a leader of the ticket effort.

Bagley, a mild-mannered legislator known for his bulldog toughness in legislative skirmishes, has wielded a strong influence in the General Assembly since his election in 1966.

In addition to his control of the state's $16 billion biennial budget, Bagley has led the legislature in several moves to upgrade the state's treatment of the mentally ill and recipients of welfare and to provide full funding for the state's share of local education costs.