Just when it seemed political events in South Carolina couldn’t turn more bizarre, calls are mounting for investigations and for the withdrawal from the Senate race of Alvin Greene, who has become known as the “stealth candidate” after mysteriously becoming the state’s Democratic nominee on Tuesday.

On Thursday, not only did more officials urge Mr. Greene to quit the contest against Senator Jim DeMint, the Republican incumbent, but top lawmakers — like Representative James Clyburn — began suggesting that Mr. Greene was a “plant.” Mr. Clyburn, the House Democratic majority whip, also called for a federal investigation because of reports that Mr. Greene, who is on unemployment, paid more than $10,000 of his own money to enter the race.

“There were some real shenanigans going on in the South Carolina primary,” Mr. Clyburn said on a radio show. “I don’t know if he was a Republican plant; he was someone’s plant.”

The first startling disclosure was made by The Associated Press, which reported that Mr. Greene had been charged with a felony in a case involving showing pornographic images to a college student. Mr. Greene, 32, was largely silent about that on Wednesday as more and more media tried to contact him and officials began calling for him to withdraw from the race. (In interviews on Thursday, he wouldn’t comment on the charge, saying he was innocent until proven guilty. And he said he wouldn’t step down.)

Carol Fowler, a state Democratic Party leader, revealed in interviews that she had questioned Mr. Greene’s use of a personal check for that $10,400 for filing fees to enter the race.

The Root.com and some local TV stations have now published and broadcast interviews on Thursday in which Mr. Greene, who says he has been unemployed since he was discharged from the military nine months ago, speaks in platitudes about his campaign and contends that he traveled the entire state to get his message to voters. But, he’s not specific at all in the interviews about where or how he campaigned. He even concedes in one of the television interviews that he never held a campaign event, didn’t attend any larger Democratic primary gatherings and didn’t even have a campaign Web site. And he never filed any financial reporting forms.

Asked how he won, he said South Carolinians could relate to him more than any other candidate.

Mr. Greene was also asked about the cash he used to pay the filing fee, especially given the fact that he’s receiving unemployment benefits and also is represented in criminal court by a public defender. He said it was his own money.

But Mr. Clyburn’s remarks fueled even more suspicions after The Hill newspaper published what the lawmaker had said on the Bill Press radio show. “I would hope the U.S. Attorney down there would look at this,” Mr. Clyburn said. “Somebody gave him that $10,000 and he who took it should be investigated, and he who gave it should be investigated.”

That seems to be the growing consensus among state and local officials. The Post and Courier in Charleston interviewed several puzzled lawmakers and Mr. Greene’s Democratic opponent for the nomination, Vic Rawl, a former state lawmaker. (He spent about $250,000 on his campaign for the Senate nomination, and didn’t take Mr. Greene’s candidacy seriously until he began winning Tuesday night, the newspaper notes. (Mr. Greene received 59 percent of the vote.)

So how did this come about? Was it his top ballot position? The fact that he’s African-American, which some have suggested might have swayed the sizable numbers of black primary voters? Or was it something else?

Here’s what the Post and Courier reported from its interviews:

State Representative Bakari Sellers, D-Denmark, suggested Greene might have benefited from being listed first on Tuesday’s ballot, but Fowler said the party’s two relatively little-known Senate candidates in 2008 polled at nearly 50-50. State Senator Robert Ford, D-Charleston, who lost his gubernatorial bid Tuesday, said race could have played a role. The Democratic primary electorate is majority black, as is Greene, but not Rawl. “Vic Rawl had money, but he didn’t have enough. He wasn’t able to identify himself with black voters,” Ford said. “No white folks have an ‘e’ on the end of Green. The blacks after they left the plantation couldn’t spell, and they threw an ‘e’ on the end.” If outside forces played a role in Greene’s win, they’ve covered their tracks well so far. The race didn’t seem to tilt based on any last-minute mailing, robocall or word of mouth through established Democratic networks. “Whatever happened, it was damn stealthy,” Rawl said.

Top Democratic officials in the state are noting that tomorrow (Friday) is the day the election returns must be certified. If Mr. Greene were to withdraw, Mr. Rawl could easily be named the nominee, according to news reports.