The Senate Governmental Affairs Committee hopes to convince the Reagan Administration to tone down proposed changes in federal pay and layoff rules scheduled to go into effect sometime this fall.

If the changes are made, an employe's job security during a reduction-in-force would be based on his latest job performance rating rather than his seniority.

The administration also wants to change the rules governing within-grade pay raises (worth 3 percent) that go automatically to 99 of every 100 eligible workers. That change would make employes meet new uniform performance standards to get the within-grade raises and also to qualify for speedy promotions in certain fast-track career fields.

Federal employe unions have attacked the performance-over-seniority changes. They say this system would make it too easy for political bosses to punish uncooperative aides by giving them a bad performance rating before they came due for an in-grade raise, or to dump a longtime worker with a good career track record by giving him one bad rating and then running a RIF.

Administration officials say the emphasis on performance conforms to the practice in the private sector. They contend it would save the jobs of outstanding women and minorities who are often the first to be riffed because of the government's last-hired, first-fired regulations.

Sen. Ted Stevens (R-Alaska) wants past performance (not just an employe's most recent rating) to be considered in deciding who gets a raise, or who gets riffed. He hopes to get the administration to agree to it during upcoming hearings on a bill to revamp the merit-pay system already in effect for supervisors and managers.

That bill, by Sen. Paul Trible (R-Va.) and Rep. Frank Wolf (R-Va.) would revise the merit-pay system that covers 150,000 managers/supervisors in Grades 13, 14 and 15. If Stevens can't bargain for changes in the pending performance ratings rules, he may make them part of the managers merit-pay reform bill.

On the House side, Rep. Patricia Schroeder (D-Colo.) has introduced a bill to freeze the administration's pending job changes until Congress has had a chance to consider them. Rep. Steny Hoyer (D-Md.), a member of the powerful Appropriations Committee, may try to offer a legislative rider on an appropriations bill that would bar any changes in pay or RIF rules.

The comment period on the proposed changes in RIF and performance regulations ends May 31. Administration officials hope to have a new system in place by October. That is the start of a new fiscal year, and just about the time agencies that may be planning RIFs would begin them.