President Clinton today pledged to stand by Henry Foster, his embattled nominee for surgeon general. "I support him," said the president. "I want him to have his hearings. I believe the Senate will support him. We should not back away from this." Prominent Democrats, including Senators Barbara Boxer and Dianne Feinstein of California, accused the pro-life movement of "stalking" and "terrorizing" Foster. His nomination came under increasing fire when, last night on ABC-TV's "Nightline" program, he said he'd performed 39 abortions during his 38-year career, more than three times the number he first cited a week earlier. He added that he had no intention of withdrawing his name, and denounced abortions as acts of failure.TIME Washington correspondent Nina Burleighgives Foster barely a 50-50 chance. "The Clinton Administration never fails to underestimate the ferocity of its opponents," she says.. Still, expect a bloody fight to the finish: "The Administration can't afford to back off on Foster." says Burleigh, "because they've caved in so many times."