Congress and the White House are about to get fiscal over just who runs the people who run the U.S. government.

The Reagan administration says it has the authority to say what hurdles white-collar civil servants must leap to get pay raises, promotions and to keep from being fired when Uncle Sam tightens his belt. It wants to downplay the value of seniority and let performance decide tenure, raises and promotions.

Accordingly, the Office of Personnel Management says that, come sundown Aug. 1, it will start making those changes unless Congress officially says otherwise.

House Democrats are seeking to bar the changes at least through September 1984. The House has voted to ban any changes (at least until Oct. 1) as part of the Treasury-Postal Service-general government supplemental money bill to tide many agencies over through the remainder of this fiscal year.

The House Appropriations Committee last week voted to extend the rules-change ban through the 1984 fiscal year. The full House is expected to okay that ban.

The White House has warned Republican House members the president will veto any supplemental spending bill that prohibits Reagan from making the changes. The Senate is working on the same supplemental bill, but minus the House language telling Reagan hands off the civil service.

Administration officials are counting on the Senate to insist that the president be free to make the work rules changes. But they are likely to lose the support of Republicans such as Charles McC. Mathias of Maryland, Paul S. Trible and John W. Warner of Virginia and Ted Stevens of Alaska. They represent big groups of U.S. workers who fear the proposed changes primarily because they distrust the motives of the people seeking the changes.