CANADIAN Prime Minister Brian Mulroney might reasonably expect one of those dark clouds swirling round his head to have revealed its silver lining by now. But when the thunderclouds lift it's only to let Mr. Mulroney see the next twister coming at him.The prime minister's approval rating is now down in the low teens - a number smaller than Wayne Gretzky's skate size. The voting base of his ruling Progressive Conservative Party is threatened in Quebec by the separatist Bloc Quebecois and in Western Canada by the right-wing populist Reform Party. Canada's economy is still in recession and unemployment is still rising in many regions. And Canadian shoppers are now spending billions of dollars just across the border in the United States to beat northern taxes. The national unity crisis looms even larger. After years of painful, complex, and profoundly frustrating constitutional negotiations, many French-speaking citizens of Mulroney's native Quebec have decided they would rather walk than talk. They say Canada has failed them and they want out. Western Canadians, too, are feeling profoundly disaffected; many politicians say they are tired of Quebec's threats and whining and they want Quebeckers to decide once and for all to learn to love Canada, or to leave it . It was hard to imagine how things could get much worse - especially for Mulroney, who still hopes to hold the country together and be re-elected within the next two years. Then Glen Kealey came along. For Mulroney and his Tory party, Mr. Kealey has unquestionably made things much, much worse. And that's just exactly what Kealey wanted. In late 1989, Kealey, a former printing company executive and would-be developer, went before an Ottawa justice of the peace and charged that three of Mulroney's cabinet ministers, a lobbyist close to the prime minister, and three government aides were involved in a bribery and kickback scheme. He also alleged that three senior Mounties were blocking an investigation of the crimes. It was a story Kealey had been trying to get people to listen to for years. But most everybody - including the major media - wrote him off as a kook. That all ended this summer when Justice of the Peace Lynn Coulter approved criminal charges against a total of 16 people on the basis of Kealey's complaint. The cases are set to come to trial Sept. 16. Kealey claims that he was asked to pay a bribe and arrange a kickback on a planned office development by one of Mulroney's cabinet ministers in 1986. He says that because he refused to pay, he was denied federal support for the $160-million project and it collapsed. Further, he claims that this was not an isolated case of a single corrupt official. Kealey claims that elements within the Conservative Party itself are running a multi-billion dollar kickback scheme on most federal government spending in Quebec. OR the past two and a half years, Kealey has been picketing the Canadian House of Commons and screaming insults and accusations at the prime minister and senior members of his party. This quixotic campaign against alleged corruption has cost Kealey his business, his house, his marriage, and, from time to time, his freedom. He has been arrested eight times - on such improbable charges as shouting in the street - and has served five days in jail. But his courtroom success has made Kealey an instant hero in the eyes of the many Canadians who distrust the Mulroney government. Thousands of dollars have already been donated to his cause and he is now organizing a national speaking tour. In Ottawa, he has opened an office and is incorporating as the Canadian Institute for Political Integrity. If the Ontario attorney general proceeds with the charges - and he must, should his investigators find any basis for them - the federal government and Mulroney will be terribly damaged. The only item on the agenda of the Mulroney government now - the only item that can possibly save it from political oblivion - is restoring national unity. That means selling English-speaking Canadians on a more generous deal for Quebec and selling Quebeckers on a deeper commitment to Canada. But Mulroney can hardly lead a divided country into a new era of hope and fidelity while his friends, cabinet ministers, and former associates - almost all of them from Quebec - appear in the courts accused of a deeply cynical abuse of public trust. Kealey's charges may well further exacerbate anti-Quebec feelings and strip from the tattered Tories the last shred of political legitimacy. That will delay and confuse the process of reconciliation in Canada. It will also heighten the appeal of the English-speaking Canadian populists and Quebecois separatists who have simple but extreme answers to the complex and continuing problems of Canadian unity.