House Rules Committee Chairman Claude Pepper (D-Fla.) yesterday suggested a one-year delay in mandatory Social Security coverage for new federal employes in order to reduce government workers' opposition to the $165 billion Social Security rescue package.

Pepper told the Senate Finance Committee that, unless the revenue loss would be too great, the date for putting all new federal hires under Social Security should be postponed from next Jan. 1 until 1985. This would give Congress an extra year to work out a supplementary government pension plan on top of Social Security, similar to plans in private industry, for those workers affected by the proposal.

Finance Committee Chairman Robert J. Dole (R-Kan.), like Pepper a member of the presidential advisory commission that recommended the rescue plan, said he would look at the idea to "see what we lose." But sources said support for postponement is not very great in the House Ways and Means Committee, which is to begin voting next week.

At his news conference last night, President Reagan said it "ill behooves" federal workers to oppose a plan that is compulsory for "all the people in the rest of the country." He pointed out that their present pension plan is funded largely from general revenues rather than individual contributions, and added: "I don't see where they can say there's any threat to the existing program in the newcomers."