D.C. City Council members who lost a fight to extend the city's tough rent control law blamed Mayor Marion Barry yesterday for not working hard enough to defeat provisions approved by the council Tuesday that weaken controls.

Despite passage of the new bill, tenants' groups vowed that the fight for a stricter rent control law was not over and said they would take the issue to the general public in the form of an initiative they will try to get on the ballot in the fall.

"The mayor didn't spend one chip," said City Council Chairman David A. Clarke, who had introduced a bill to extend the old rent control law for four years. "The mayor watches to see which way the wind is blowing on the council . . . . The mayor could have swayed votes."

Clarke went into the battle with six votes, only one short of the seven needed for approval.

But, in the end, a majority of the 13-member council voted for a measure that will allow 12 percent rent increases on apartments that become vacant, instead of the current 10 percent, decontrol buildings that are 80 percent vacant and exempt from controls single-family homes owned by individuals when the current tenants move out.

Barry, who had supported a six-year extension of the old rent controls, late Tuesday signed the new rent control bill and transmitted it to Clarke with a letter saying he was doing so "despite provisions which cause me grave concerns." The previous law expired Tuesday night.

The mayor repeated his opposition to decontrolling single-family homes as they become vacant and a phase-out of controls that could begin in 1989 if the city's apartment vacancy rate exceeds 6 percent and the city has a rent subsidy program. The current vacancy rate is about 2.4 percent.

"Already the executive has begun to calm the anxieties of tenants, fearful that they will be forced out of their units," Barry said. He added that the final bill as passed "remains a strong rent control law, one of the strongest in the nation."

Clarke said council members supporting continuation of the current controls had asked the mayor to reiterate his stand forcefully on the controversial provisions they had hoped to change. But the mayor failed to do so, Clarke said.

"Too little. Too late," said the three-sentence letter signed by Clarke and five other council members responding to Barry's transmittal letter.

Clarke said yesterday that putting the rent control issue to a popular vote could be one effective way of getting around Tuesday's council action, but he said he would have to see a proposed initiative before endorsing it.

Jim Henderson, chairman of the Emergency Committee to Save Rental Housing, a coalition of tenant and community organizations, said his group has not yet decided whether to endorse a rent control initiative already proposed to the D.C. Board of Elections by James Clark, a city resident, to extend the old law for 10 years, or whether to propose another.

Henderson said a tenants' political action committee has been formed to oppose reelection of three council members running next year who supported the bill that passed: Nadine Winter (D-Ward 6), William Spaulding (D-Ward 5) and Betty Ann Kane (D-At Large).