Republican candidate Bill Simon on Wednesday conceded his allegation that Gov. Gray Davis illegally accepted a campaign check inside the state Capitol "is now in question" and Davis called on him to drop out of the governor's race.

But Simon defended himself for making the claim, which was based on now-discredited photographs released by a law enforcement group that is a key Simon ally and a complaint the group made to a state watchdog agency.

"I had no reason to question the California Organization of Police and Sheriffs' good faith belief in their charge against the governor," Simon said in a statement.

"...They were reporting the charge directly to the government enforcement agency, and they claimed that they had documented proof of their charge."

Simon made the accusation Monday after the first gubernatorial debate and said he had evidence.

The evidence turned out to be two photos COPS released Tuesday they claimed showed then-Lt. Gov. Davis accepting a check in his Capitol office during his first run for governor in 1998. It's a violation of state law to give or receive campaign contributions in state buildings.

Scrutiny by reporters and the Davis campaign showed the photos were not taken in Davis' office.

Simon did not back completely off his allegation.

"The location where the governor received this campaign contribution is now in question," his statement said. "However, even if the specific claims made by the California Organization of Police and Sheriffs are not sustained, this outcome should not deter the Fair Political Practices Commission, other law enforcement agencies and the media from investigating Gray Davis' aggressive and shady fund-raising practices."

Davis, meanwhile, called on Simon to withdraw from the race for making the allegation.

"He should drop out," the governor said on KGO-AM.

"This whole sorry episode has backfired in Mr. Simon's face ... as a former prosecutor, he should certainly be embarrassed, and if he had any sense of honor, he would drop out of the race. You have to check your facts in this business," Davis said.

Davis said his campaign would consider suing COPS.

"We will look into our legal options, because this is definitely going over the line," he said.

The nearly identical photos show Davis standing in an office next to Al Angele, then executive director of COPS. Both men are smiling and holding a corner of a $10,000 check COPS gave Davis.

Monty Holden, current executive director of COPS, which broke bitterly with Davis to back Simon, told reporters Tuesday that Angele and Davis were in the lieutenant governor's office.

That proved not to be the case.

The office pictured in the photographs bore no resemblance to the lieutenant governor's office. Angele said he'd never set foot in Davis' office. Davis said he was not even in Sacramento when the photos were purportedly taken.

Simon acknowledged his campaign, which has donated over $200,000 to COPS to appear on its slate mailer, had not sought to verify the authenticity of the photographs.

It remained unclear Wednesday where the photos were taken. Davis said he wasn't sure but it was presumably at a private office where COPS took a picture to use in their newsletter.

The episode is the latest embarrassment for Simon, a first-time candidate who lags behind Davis in polls and has suffered a series of setbacks, including a civil fraud verdict against his family investment firm that was later thrown out by a judge.