The sponsor of a bill to curb "mansionization" in Montgomery County is threatening to withdraw his support, saying that a committee chaired by County Council member Steven A. Silverman has delayed and weakened the measure to the point that it might do nothing to limit heights of new houses in older neighborhoods.

The bill, introduced two years ago by Howard A. Denis (R-Potomac-Bethesda), is designed to eliminate loopholes and ambiguities in the law that have allowed some builders to exceed the limits of 35 feet and 21/2 stories.

The proposal has been watched closely by the building industry, which wants minimal restrictions on the large, multistory houses sprouting up in neighborhoods across the county.

The bill will expire if the council does not act on it by the end of the year. To be reconsidered, it would have to be reintroduced in 2006, as local elections heat up.

"This bill has gotten more convoluted scrutiny than any other zoning text amendment in the history of Montgomery County," Denis said yesterday. "That tells me I must be on to something."

Silverman (D-At Large), chairman of the council's Planning, Housing and Economic Development Committee, which will discuss the bill tomorrow, said he has not delayed or weakened it. He said other matters, such as the budget and affordable housing, have taken priority.

"I was hopeful, perhaps naively hopeful, that agreement would be reached among the parties," he said.

Denis's bill has remained in Silverman's committee for almost two years, a period in which Silverman has clearly signaled plans to run for the Democratic nomination for county executive. He has built a campaign treasury largely on the strength of contributions from developers and businesses that benefit directly from land development, according to an analysis by Neighbors for a Better Montgomery, a watchdog group.

When he ran for the council in 2002, Silverman received 60 to 70 percent of his $265,081 in campaign funds from development interests, the group said. As of January, his campaign had collected $585,663 from such interests. Denis has received substantial funds from the same sources, accepting $177,589 in the 2002 race, Neighbors for a Better Montgomery said.

Silverman said he has not gone out of his way to cater to developers.

"I have been incredibly fair in the handling of our work sessions," he said. "It continually disappoints me that people go after somebody's motives rather than debating the merits of the issue."

The situation has grown so tense in recent days that Denis, a seasoned Capitol Hill staffer and former state senator, threatened Silverman with a "discharge petition," a seldom-used procedural maneuver employed by members of Congress to get their bills to the floor. It has never been used in Montgomery, Denis said.

The push to limit the heights of houses occurs as the county's success at regulating growth has come under fire. Allegations that builders have violated height limits and setback requirements for new houses in Clarksburg and Bethesda have triggered investigations and hearings.

At the same time, rapid mansionization of the county's older neighborhoods has raised concern that there is little scrutiny of "in-fill" development by the county government.

Marilyn Praisner (D-Eastern County), a member of Silverman's committee, said she found the amount of time the panel has spent on the bill somewhat puzzling. "Let's stop going around and around," she said yesterday. "Let's move it."

Current law places some restrictions on construction of new, large houses, requiring in many cases that builders put eight to 10 feet between a new house and neighbors on both sides.

But the law also makes what would seem to be a fairly straightforward matter -- measuring the height of a house -- almost impossibly convoluted. There are ongoing disputes over what constitutes a story, when a basement is not a basement and where measurement of the 35 feet should begin.

Builders also have been able for several years to rely on a county regulation that allows them to tear down almost all of an older house except two exterior walls. They can then label the new construction an addition or renovation and avoid stricter requirements on new houses.

Denis's bill would simplify the way heights are measured.

Carol Green, an environmental lawyer who lives in a section of Bethesda where older houses are rapidly being supplanted by towering, multistory dwellings, said lack of strictly enforced height limits has given builders free rein.

"The character of lovely older neighborhoods has been changed as a result of very clear loopholes in the zoning ordinance that everyone admits exist," she said.

Green and other neighborhood activists had hoped Denis's bill would begin to close some of these loopholes, but they are now worried it could have almost no effect on building heights.

"It may be worse than no bill at all," said Barbara Siegel, a Bethesda architect and community activist who worked with Denis to craft the original bill.