Amid denunciations of one of the creatures most unpopular among Maryland politicians - the white-collar crook who cooperates in political corruption probes - the Maryland Senate today rejected a proposed amendment that would encourage such cooperation.

The amendment was offered to a bill that would disqualify from state contracting any person or corporation convicted of bribing a state or county official. The amendment would have exempted persons who cooperate with prosecutors from this disqualifications.

"To reward this person, who consciously bribed a state official, to say he should not be penalized at all, because he was the coward, the one who didn't want to go to jail . . .

It's abhorrent," said state Sen. Edward T. Conroy (D-Prince George's).

The same theme echoed through the comments of several other senators who spoke agaisnt the amendment.

"The guy who cooperates with the prosecutors is not so much concerned with continuing to do business with the state as with saving his own neck," thundered Sen. John A. Cade (R-Anne Arundel).

"We're not talking about some poor guy who suddenly has a pang of conscience," said Sen. Clarence Mitchell III, arguing that the amendment would encourage businessmen to bribe public officials, "We're talking about a crook."

Both the bill and the amendment were sponsored by Sen. John Carrol Byrnes (D-Baltimore), a former prosecutor. In the midst of a speech in favor of his amendment, Byrnes said the bill as it now reads "would kill, perhaps fatally and permanently," the option of a coconspirator in a bribery scheme to cooperate with the state.

Byrne's reasoning was not convincing to his colleagues, who voted 21 to 25 to defeat the amendment. Loyalty is the single most important at tribute most Maryland politicians look for in their associates; the idea of helping someone who turns on his one-time associates is anathema to many of them.

A number of Maryland politicians have been convicted in recent years thanks in part to testimony provided by cooperating coconspirators. Most of these have been federal prosecutions. Among the politicians have been former Vice President Spiro T. Agnew, Baltimore County Executive Dale Anderson and Anne Arundel County Executive Joseph W. Alton.

Almost neglected in the debate over the amendment were the legal sanctions against the briber who turns state's evidence. The Senate debate was over allowing that person to get state contracts in the future his fate at the hands of the courts was not spoken of in the Senate.

Most of the support for the bill came from senators concerned about potential witnesses' willingness to cooperate with investigations into alleged corruption. Without the amendment, said Sen. John J. Bishop, Jr., (R-Baltimore County), the bill is "not only chilling, it's freezing," in its effect on potential witnesses.

"Without this amendment, this bill is really to protect the corrupt politician," said Sen. Julian Lapides (D-Baltimore). "I think we have to allow a person the right to cooperate with the state's attorney or the U.S. attorney."

"You're making it an awful lot easier" for people to contemplate bribing state officials, said Sen. John P. Corderman (D-Western Maryland), "because their coconspirators turning against them."