The Senate is expected to vote today on a controversial proposal to wipe out an important pension tax benefit for more than 20 million government and postal workers and others who contribute to their retirement funds.

Sen. Paul Trible (R-Va.) plans to offer an amendment to tax reform legislation that would protect the right of new retirees to recover previously taxed contributions without having to pay taxes a second time.

They currently have up to three years to recover those contributions before they resume paying taxes again. For the typical federal retiree, it takes about 18 months.

The House-passed tax reform bill would eliminate the recovery rule for persons who retire after July 1.

The tax reform bill before the Senate today would phase out that benefit in two stages, beginning in January 1988.

Once the Senate approves a tax reform plan, it will attempt a compromise with the House.

If the House version prevails, employes retiring after this month would be required to start paying taxes on the government contribution to their pension plans.

The amount taxed would be prorated over the life expectancy of each individual.

Trible wants the Senate to strike the pension tax change from its bill so that opponents of the change will be in a stronger bargaining position when it gets to the House-Senate conference.

Most federal and postal unions and groups representing supervisors, managers and executives oppose the pension tax change.

The Federal Government Service Task Force says that while the pension tax change would not increase the lifetime tax burden of retirees, it would require them to begin paying taxes much sooner.

The congressional civil service caucus says such a change would mean a tax bite of $10,000 in the first three years for the typical retiree, and up to $40,000 for top-paid executives.

Smoking Ban

The Tobacco Institute said yesterday that a survey of 400 federal workers indicates that eight out of 10 government employes here would oppose legislation to ban smoking in U.S. offices and favor voluntary programs to limit on-the-job smoking.

The trade organization, which represents major cigarette manufacturers, commissioned its poll shortly after the General Services Administration announced plans to restrict smoking in most parts of federal office buildings.

The firm of Fingerhut Granados Opinion Research polled 400 federal workers here by phone. There are about 350,000 federal employes in the Washington area.

The polling firm said 28 percent of the respondents identified themselves as smokers, 18 percent as former smokers and 54 percent as nonsmokers.

According to the poll, 77 percent of the nonsmokers said they rarely had a problem with coworkers who smoke, and 61 percent said there is no need for new rules to limit smoking.

Eight of every 10 respondents opposed a no-smoking law.

In addition to the GSA rules, which limit smoking to private offices and designated areas, several bills are pending in Congress that would further restrict smoking or ban it on government property.