Volunteers in Patrick J. Buchanan's presidential campaign on Sunday were soliciting members of Ross Perot's Reform Party to re-register as Republicans--the first sign of serious on-the-ground campaigning in the California primary and the first firm evidence of cooperation between Perot and Buchanan forces.

About half of the Reform Party members being contacted expressed a willingness to change their registration so that they could support Buchanan with their votes in the state's March 26 primary, said Penny Ferguson of San Mateo, Buchanan's volunteer coordinator for San Mateo County.

There was no indication of how many new voters the Buchanan campaign might pick up by today's 5 p.m. registration deadline, but the move was considered a significant indication of a potential alliance between Perot and Buchanan, who have taken some similar positions and whose political bases of support overlap to a considerable degree.