A few wavering Democrats could control the outcome tonight when Allegheny County Council returns to a refurbished Gold Room at the county courthouse for a vote on Chief Executive Dan Onorato's second assessment plan of the year.

"Right now, I'm up in the air," said Councilman Rich Nerone, D-Brookline. "I'm not sure if I can make an informed decision."

A Carnegie Mellon University professor performed a statistical accuracy test of the new assessment values, posted on the county Web site last month, and found that they do not meet internationally accepted standards when compared to recent sales.

Mr. Nerone and several other council members have requested that Mr. Onorato show them how his newest batch of assessed values for 2006 -- with a 5.8 percent average increase for residential property countywide -- would affect each of the county's 130 municipalities. The chief executive said his office would try to provide that information today.

In March, all of council's eight Democrats and four of its seven Republicans voted to approve Mr. Onorato's plan to put a 4 percent cap on assessment increases. Without the cap, assessments would have increased an average of 19 percent for the county's 500,000 residential properties.

A judge rejected the cap, and the chief executive decided to revise values from the last reassessment, in 2002, to limit the size of increases.

Tonight, however, Republicans say they will vote against Mr. Onorato's bill because of concerns about its legality. Some Democrats seemed wary about giving their support.

"How does it affect my specific constituents?" Mr. Nerone said. "I think the prudent thing to do would be to send this back into committee."

Republicans are proposing an alternative plan: use the March numbers that Mr. Onorato discarded and set the assessed value at 80 percent of market value.

Mr. Onorato met with some Republicans yesterday to discuss the plan. He has said it would lead to hefty increases in some parts of the county.

Mr. Onorato also said he planned to meet with Democrats today to try to assuage their concerns. Democrats Bill Robinson, Chuck Martoni and C.L. Jabbour have said they want more specific information from the Onorato administration.

"I haven't decided yet, but I believe Dan Onorato's approach is a good approach," said Mr. Robinson, of the Hill District. He also said he would consider the Republican plan.

Mr. Jabbour, of West Mifflin, said Republicans should have proposed their plan sooner, but Councilman Doug Price, R-Carnegie, its architect, said he didn't start considering alternatives until Mr. Onorato unveiled his new proposal.

"Once I saw how bad it was," said Mr. Price, "I had three options -- vote for it, vote against it, or come up with something else."

Mr. Onorato's new numbers do not meet statistical standards set by the International Association of Assessing Officers when compared to recent sales, said Robert Strauss, a professor of economics and public policy at Carnegie Mellon University who has sparred with the chief executive over assessments in the past.

Mr. Onorato said Mr. Strauss has "no credibility" on the issue because he lives in Westmoreland County, which hasn't done a reassessment since 1973.

"He expects Allegheny County to live under a different standard," Mr. Onorato said.

Mr. Onorato also said sales prices played a smaller role in developing his new assessment numbers than it did in typical reassessments, a fact that lessens the value of statistical tests for accuracy like the one Strauss did.

Despite the chief executive's reassurances, some council members still are uneasy, and council President Rich Fitzgerald, D-Squirrel Hill, has said he would be willing to send the issue back to committee if he can't muster enough votes to pass the plan tonight.