With just three weeks to go before the election, the National Conservative Political Action Committee (NCPAC) announced yesterday that it will pour another $75,000 into its half-million-dollar campaign against Sen. Paul S. Sarbanes (D-Md.). The right-wing group unveiled its latest anti-Sarbanes commercial -- a 10-second spot on busing that will air on Washington and Baltimore TV stations.

NCPAC's ad opens with a frightened-looking little girl, clutching her school books, standing inside a school bus doorway. "On election day remember, Paul Sarbanes voted for busing," a voice intones, as the bus door slams shut. As the bus drives off, the voice says, "But [he] sent his child to a private school."

Meanwhile, Sarbanes, campaigning in Silver Spring before retired federal workers, drew cheers when he blasted NCPAC, shouting "misrepresentations and negativism have no place in Maryland politics."

Later, he told a reporter that NCPAC "has been distorting my record on busing since they launched their attack. I'm not for forced busing, I've never voted for it." But Sarbanes said he has voted against measures that would prohibit the courts from issuing busing orders and the Justice Department from enforcing them.

NCPAC has made the liberal Sarbanes its No. 1 target, and with its latest ads will have spent about $625,000 to defeat him. The group has shown ads blasting Sarbanes and applauding the record of Republican challenger Lawrence J. Hogan.

But Sarbanes, some say, has transformed the NCPAC campaign from a curse to a blessing, by making it his biggest issue. Yesterday, Democratic Rep. Michael Barnes, campaigning with Sarbanes, drew his biggest applause when he told the retirees that the election would give them a chance "to reelect Paul Sarbanes . . . and kick NCPAC out of Maryland."