INDIANAPOLIS (AP) _ A man in custody today on charges of planting 13 bombs around his southern Indiana hometown last summer may have been targeting a state lawmaker, authorities said.

John M. Hubbard III, 31, was arrested Wednesday with Jerry A. Conrad, 42, a former co-worker in Salem, said U.S. Attorney Deborah J. Daniels. Both were being held without bond at the Marion County Jail.

Although she wouldn't discuss a motive, Daniels said state Rep. Frank Newkirk Jr., the 32-year-old son of Salem's mayor, may have been a target.

''Hubbard told other people that he felt very strongly about Mr. Newkirk and that he hated him,'' she said. ''He didn't say why.''

A grand jury was scheduled to convene today in Indianapolis to consider 29 preliminary charges against each suspect: 14 counts of manufacturing destructive devices, 13 counts of attempting to damage property, one count of conspiracy to manufacture destructive devices, and one count of unlawful possession of stolen explosives.

Convictions on all counts could bring maximum penalties of 285 years in prison and $7 million in fines.

During a hearing in New Albany, a federal magistrate granted a government motion for a psychological examination of Hubbard and set a May 31 competency hearing.

The motion said Hubbard ''has a history of emotional problems stemming in part from substance and alcohol abuse.''

Hubbard is ''a self-styled 'survivalist' who has stated that he is 'armed' for a 'confrontation' with the police or anyone else, and that he will never go to jail, even if convicted at a trial,'' the motion said.

''In December 1988, Hubbard twice telephoned the Nicaraguan Embassy in Washington, D.C., asking for political asylum; Hubbard claims to be personally acquainted with Daniel Ortega,'' the Nicaraguan president.

The bombs were found July 5 around Salem, a town of 5,500 residents. Seven bombs, which were in paper grocery sacks, had been stashed under bridges. The remainder were hidden near businesses and government buildings.

None exploded. Three burned, causing minor damage.

Hubbard, who went to school with Newkirk, considered the state representative to be ''a lifelong rival,'' according to an affidavit.

''There are indications in the affidavit that John Hubbard bore some animosity toward Frank Newkirk Jr., and bore some animosity toward an employer of his,'' Daniels told reporters in her Indianapolis office.

Newkirk acknowledged that he may have been the target.

''I know from his comments in the newspaper and from our conversations he considers himself to be a very liberal person and I'm pretty conservative,'' he said. ''Also, I believed from our discussions that he has a dislike or resentment for government, and I may represent government to him.''

Newkirk said he and Hubbard occasionally spoke when they met on the street, but he does not know Conrad, who worked with Hubbard at Ferraloy Industries in Salem.

''This has been a real stressful time for our family, and we're very relieved that the arrest has been made,'' Newkirk said.

Newkirk noted that he was married 11 years ago Wednesday and called the arrests ''a nice anniversary present.''

Ron Marker, whose sister found 25 sticks of dynamite wired to explode behind the family service station, said the arrests brought some relief to Salem.

''It's been going on for a long time,'' he said. ''I hope they got them.''