Administration officials assigned to sell the president's ambitious, and controversial, plan to "reform" the government's $60 billion pay machine are walking a narrow credibility tightrope.

On one hand they want to convince the big federal bureaucracy, union leaders and politicians with big federal voter blocs, that nobody working for Uncle Sam would lose a nickel if pay reform becomes law Indeed, they hint that many big city federal workers might get larger pay raises in the future if unhooked from their country cousins, and matched instead with local, hometown industry pay scales.

Yet when talking with groups less sympathetic to the federal work force, the Carter people point out that reform could shave an estimated $3 billion from federal pay raises over the next few years. That savings is described as a "cost avoidance" (a term popularized by former Defense Secretary Robert McNamara back in a simpler era). But the implication is clear to governors, city managers and business types who believe their own workers get ideas every time Uncle Sam raises pay rates.

The Carter plan would eliminate across-the-board nationwide pay rates for many, if not most, federal jobs.Instead of comparing specific federal job and grade levels with a narrow band of industries, it would match up federal pay and fringes against industry, in a so-called "total compensation" package. Idea is that government fringes are worth more, so pay raises in future would be smaller.

What strikes fear most in the hearts of federal union leaders is the Carter proposal to compare and match up federal salaries with pay for the same jobs being performed by 12 million local and state government employes. In most cases the local government people earn less than their federal counterparts, and that too could shave future raises for civil servants.

Carter also wants to link federal pay rates on an area-by-area, or city-by-city basis to salaries and fringes in hometown industry. For some federal workers that could mean larger raises, but in smaller towns and in cities where Uncle Sam is in the pay vanguard, it would slow the rate of raises for civil servants, and lessen pay pressures on private employers.

Many experts believe the White House will have a tougher time selling Congress on pay reform than it did on civil service job reforms. Perhaps.

But the Carter people realize that most members of Congress are not named Fisher, Spellman, Barnes or Harris. Like evangelists before the gas crunch, Carter, salespersons are pushing a major grass-roots crusade. They are doing a lot of traveling, and a lot of talking to business groups, chambers of commerce and others. And they are coming back with politically negotiable scalps in the form of endorsements from "back home" for pay reform.

The gospel of "cost avoidance" may not soothe federal workers. But the idea of "neutralizing" the impact of federal pay sounds good to many employers -- from New York Government Hugh Carey to the president of a small-town shoe company.