ELECTIONS . . . DEM-IN-CHIEF HITS BACK, WITH ADS

President Clinton, launching a last-ditch, nationwide campaign swing for struggling congressional Democrats, today flew to Philadelphia to tell elderly Pennsylvanians the GOP's "Contract with America" would slash Social Security and Medicare -- a response to a recent Republican salvo spreading the same suspicion about his Administration. To drive the idea home, the Democratic National Committee produced two new ads: One saysHouse Republican Whip Newt Gingrichonce suggested phasing out Social Security; the other seizes on Virginia Senate candidate Oliver North's remark that perhaps the Social Security system should be voluntary. "The Republicans -- They're at it again," the ads say. BTW: The Democratic Party, which just emptied its pockets on a $2 million ad campaign, is looking for donors to lay out $1 million for this one.