At issue is Colonel North's claim that what he did in supplying arms to Iran and diverting money to the Nicaraguan Contras was known to President Reagan. Virginia Republican Senator John Warner immediately accused Colonel North of betraying Mr Reagan and called on him to withdraw from the race.
An accusation by Mr Reagan that he is lying is especially damaging for Colonel North. It is difficult for him to dimisss it entirely because he is bidding for the leadership of the Republican right, which hero worships the former president. It gives a lift to the campaign of James C Miller, a former Reagan budget director, who is standing against Colonel North in the race for the Republican nomination.
At first the colonel, whose career as a right-wing standard-bearer first took off when he gave a bravura performance in the Iran-Contra hearings, seemed to have the Republican nomination wrapped up. Nobody else could imitate Mr Reagan's patriotic rhetoric so well. He spoke of his 'vision of the future, as an autumn morning on the banks of the Shenandoah'. This vision also took a concrete form in the shape of heavy donations from conservative contributors.
Mr Miller, a supply-sider economist of impeccably conservative credentials, was at first short of both charisma and money. But many Republican leaders have been increasingly dubious about having a maverick like Oliver North in their ranks in the Senate. In office he would be a constant reminder to voters of the most unsavoury episode of the Reagan years.
There is no doubt that Colonel North has suffered some damage. Last year, the Reader's Digest, an organ of the right, pitilessly detailed occasions when Colonel North had lied. The witnesses cited were all the colonel's own colleagues. For instance he had recounted how, when it became evident that the invasion of Grenada had succeeded, President Reagan had put his arms around him and said: 'I told you not to worry. You can trust Americans.' The problem is that Marlin Fitzwater, the White House press secretary, says Colonel North did not see President Reagan that day.
Almost all those involved in Iran-Contra also came out against him. Robert McFarlane, his former boss in the White House, wrote last month to the press saying: 'The word that comes closest to describing him for me is 'fanatic'.'
The fear of the Republican establishment was that Oliver North might be strong enough to win the votes of the party faithful but of nobody else. His opponent, Senator Chuck Robb, is mired in sexual scandal. Elected as a straight arrow soldier who, as a marine guard at the White House, met and married Lynda Byrd Johnson, daughter of the president, his reputation has unravelled over the last six years. A five-page letter sent to fellow Democrats last week about the exact state of his sexual relations with half a dozen girlfriends when he was governor of Virginia attracted widespread derision.
The only Republican candidate who might lose to Senator Robb is Oliver North. But until yesterday his celebrity and money seemed to make him proof against criticism. The only person who might bring him down was President Reagan himself. But Mr Reagan was said to be restrained by the Republican Party's Eleventh Commandment: 'Thou shalt not speak ill of a fellow Republican'. Now he has spoken and Republicans will wait to see if Col North's candidacy will begin to disintegrate.