The Montgomery County Council yesterday withdrew its request for a change in state law to permit a 50 percent increase in the county's income tax rate, conceding that 10 days before a primary election is a politically poor time to start talking about raising taxes.

"Although the proposal was very rational, this is the silly season." said council member Esther Gelman. "One week before the election is the worst time to bring it up. We'll have to wait for another season."

The council shifted its agenda to take up the matter yesterday to quiet the political stir created among Democratic Party leaders and incumbent legislators when the council made the income tax proposal last week.

Just in time to meet a deadline for making requests of the county's state legislative delegation, the council agreed last Friday - with dissents from some of its members - to resubmit the proposal, which it is unsuccessfully submitted to the legislaters for several years.

The council asked authority from the state to increase the county's "piggyback" income tax from 50 percent of the state income tax rate to 75 percent. The council said it would apply two-thirds of the added revenue this increase would produce to property tax relief, allowing up to a 43-cent reduction in the basic property tax rate of $3.60 per $100 of assessed value.

A majority of the council has said that the income tax is more equitable than the property tax.

County Democratic incumbents in the legislature immediately chided the all-Democratic council for tossing the tax increase debate into their reelection campaigns. County Republican Chairman Joe Kennery charged that behind the proposal was a need to "pay the bill" that the "giveaway" Democrats have mounted at taxpayers' expense "for years."

And in an effort to kill the idea indefinitely, five of the county's seven state senators said at a press conference yesterday there is no evidence that the income tax is more equitable. They said they would not support any tax increase "at this time or in the forseable future."

In other business yesterday, the council received a list of six sites that County Executive James P. Gleason has recommended for a new plant or plants to take care of the county's projected sewage treatment needs through the early 1980s.

Three of the sites are in the Senate area, two are in the Potomac-Cabin John region and one is in the Muncaster section in the northeastern section of the county. The council set a public hearing on the proposals for Oct. 5.