Early this year, Del. Walter E. Fauntroy (D-D.C.) was confidently counting on Rep. Romano L. Mazzoli (D-Ky.) to serve as a floor whip to help steer Fauntroy's D.C. statehood bill to passage this year.

Mazzoli seemed the perfect ally: a moderate southern Democrat with long years of service on the District of Columbia and Judiciary committees who commanded the respect of Republicans as well as Democrats.

But instead of rallying behind the bill, Mazzoli has sided with the Republican minority on the District Committee in criticizing the measure and voting in favor of what Fauntroy views as crippling amendments.

Yesterday, Mazzoli supported Republican amendments that, in effect, would strip the District of authority to operate Lorton Reformatory in Fairfax County and replace a statehood constitution recently approved by the D.C. Council with a far more controversial document approved by District voters in May 1982. Both amendments were defeated, 6 to 5.

Mazzoli delivered another blow by urging Fauntroy to withdraw the statehood bill until D.C. residents have had a chance to study both documents and until a constitutional convention is reconvened to draft a compromise. "I really think a better process would be a whole new process downtown," Mazzoli said.

While Fauntroy said he is still confident the House will approve statehood legislation this year, Republican opponents were gleeful about Mazzoli's votes, and some statehood supporters said privately that the bill is in deep trouble.

"It's going to be extremely difficult to get a majority in the House to support the bill with Mazzoli opposing it," said Rep. Thomas J. Bliley Jr. (R-Va.), a member of the District Committee.

A District government official familiar with the statehood issue complained yesterday that Fauntroy "took Mazzoli for granted" before the committee began action on the bill and, as a result, played into the hands of Republican opponents. "It's in trouble, real trouble," the official said.

Fauntroy said he was disappointed by Mazzoli's opposition, but insisted it would not significantly hamper the bill's chances because Rep. Peter W. Rodino (D-N.J.), chairman of the Judiciary Committee, and Rep. Hamilton Fish Jr. (R-N.Y.), the ranking minority member of the Judiciary Committee, have endorsed the legislation.

"I remain of the view there are no legal impediments" to passage of the bill, Fauntroy said. "Members will disagree for their own reasons, but I cannot see any valid legal or moral reasons for the objections which have been raised by anyone here."

Mazzoli said he has not decided yet whether he will vote to send the bill to the floor. Fauntroy and Rep. Ronald V. Dellums (D-Calif.), chairman of the committee, hope to finish work on the bill by next week. Regardless of what Mazzoli does on the final vote, his actions have provided Republicans with plenty of ammunition in fighting the measure.

Last week, Mazzoli raised far-ranging questions about the constitutionality of the statehood bill and voted with Republicans on an amendment that would have stopped direct federal payments to the proposed State of New Columbia. D.C. currently receives about $425 million a year in direct federal payments.

Rep. Stan Parris (R-Va.), Bliley and other Virginia officials, who disapprove of the city's prison at Lorton, contend that it would be unconstitutional for a new state to retain control over out-of-state property without that state's consent. The District also holds title to several correctional facilities in suburban Maryland, and would retain all these properties under the current bill.

Mazzoli voted yesterday in favor of an amendment introduced by Bliley to restrict the new state's land holdings to the property within its boundaries.

"This is a political document, clear and simple, and to pass it on the floor you're going to need sufficient votes," Mazzoli said. "Lorton has been a thorn in the side {of many opponents}. Adoption of {Bliley's} amendment just might cure one of the persistent problems facing it, when and if it reaches the floor."