Former California state senator Robert Nimmo will be nominated to become head of the Veterans Administration, according to a well-placed White House source.

Nimmo, 59, was a fiscal officer when Ronald Reagan was governor of California, and he served two terms as state assemblyman and one as a state senator. He is considered close to presidential counselor Edwin Messe III.

The selection of Nimmo, which may be announced as early as today, will come at a time when the Reagan administration is under increasing criticism for botching the appointments process, particularly at the Verterans Administration.

Two prominent candidates have taken themselves out of the running, and a third has been discarded amid the ideological divisions and procedural difficulties of the Reagan appointments process.

One White House source said last night that the administration was "being nibbled to death by ducks" on the VA nomination and that it was important to fill the vacancy as soon as possible.

James H. Webb Jr., who removed himself from consideration for the VA post on Monday, had sharply criticized budget director David S. Stockman for running the veterans organization in the absence of a director.

On NBC's "Today' show yesterday, Webb rapped Stockman for going to divinity school instead of, like Webb, serving in the Vietnam war. Webb is the author of "Fields of Fire," a critically acclaimed book about that war.

The failure of the administration's selection process to produce a veterans administrator has been so visible, according to one administration official, that the White House was "desperate" to fill the vacancy.

Nimmo is considered a reliable Republican who would quiet fears that the administration intends to discard the agency.