A move to veto the District of Columbia's initiative has been effectively killed in a key Senate subcommittee. This became known yesterday when a second member of the three-member panel said he will not support overturning the voter-backed gambling measure.

But a gambling opponent in the Senate, Sen, Mark O. Hatfield (R-ORE.), is still lobbying other senators and testing the waters for support of his antigambling bill in a possible showdown vote before the full Senate later this week.

The D.C. gambling initiative, passed last Nov. 4, is scheduled to become law Monday, unless it is vetoed by Congress before then. Hatfield is now deciding whether to use a provision of the D.C. home rule charter to call his antigambling measure before the full Senate.

Hatfield's resolution to disapprove the gambling bill was first referred to the subcommittee on governmental efficiency and the District of Columbia. On that committee, two of the three members, Sens. Warren Rudman (R-N.H.) and Thomas Eagleton (D-MO.), have come out against the veto bill, effectively killing it without ever taking a formal vote or even holding a hearing.

The fate of Hatfield's antigambling bill became apparent yesterday when Rudman made his position public. "I very much believe in home rule," he said in an interview. "Our state has had a gooc experience with the New Hampshire lottery and sweepstakes. I certainlly wouldn't support a resolution to disapprove" the District of Columbia's gambling bill.

The third member of that panel, Sen. Charles McC. Mathias (R-MD.), has not taken a position publicly, although he has consistently stated that he opposes gambling as a revenue source. Also, Mathias represents Maryland, the District's neighboring state where a legal lottery profits handsomely from District residents who cross the border to play that state's numbers game.