Reagan could gain a conservative ally in Canada next week

Relations between Canada and its southern neighbor could take an upswing after the Sept. 4 election - if only because a conservative is now expected to become prime minister.

Basic policy differences between the two countries - from acid rain to foreign investment - will still be there, no matter who wins. But Brian Mulroney and his Progressive Conservatives - ahead in the polls against Liberal Prime Minister John Turner - would represent a welcome change to the Reagan administration.

''There was a lot of reserve among some - but not all - members of the Reagan administration about the Trudeau (Liberal Party) government. There would be a new starting point with a Conservative government,'' says David Layton-Brown, a professor of political science at York University in Toronto. He attributed the ''reserve'' to economic nationalism and to Pierre Trudeau's being ''less than enthusiastic about supporting American foreign and defense policy.''

Washington is picking no sides in the race. ''Canadian American relations have been improving over the past few years and this is likely to go on no matter who wins the election,'' says Carroll Brown, director of the Canadian desk at the State Department in Washington. Mr. Brown says there are fewer issues to deal with and there is better communication between the two countries. The same view has been expressed by Paul Robinson Jr., the American ambassador to Ottawa.

John Turner, who in June took over from Mr. Trudeau as Liberal Party chief and prime minister, is trailing in the polls. Two public opinion polls last week showed the Tories having 48 to 49 percent of popular support.

Both the Liberal and Conservative leaders have promised to beef up Canada's defense forces. They certainly need it. Canada contributes less to NATO on a per capita basis than does any other country in the alliance except Luxembourg.

But economic issues are the main bugbear between Canada and the United States. During Trudeau's early years in office Canadian economic nationalists worried about American companies dominating Canada, especially in the manufacturing and oil and gas industries.

Here are some of the things a policy of Canadian economic nationalism brought about:

The Foreign Investment Review Agency was introduced to block takeovers of Canadian firms if they did not appear to be in the national interest. On Canadian cable television, commercials were cut out of programs from American border television stations and replaced with Canadian ads. And in 1981 the Trudeau government brought in the National Energy Program, which sought to ''Canadianize'' the oil and gas industry.

What is going to happen next?

No one is going to turn back the clock. The Liberal government had started to loosen the rules of the investment review agency. But even Mulroney is not going to get rid of it. He will rename it Investment Canada and at least get rid of the stigma for awhile. Turner would keep the agency but use it only for ''major structural transactions'' - i.e., a takeover that might cause political heat.

Both men would retain a great deal of the National Energy Policy. Turner would do it with more gusto.

Overall political observers say that, initially at least, the election of Mulroney will be welcomed by President Reagan, who incidentally is much more conservative than the Canadian Conservative leader.

''There will be a lot less confrontation,'' says John Carroll of the University of New Hampshire, who has written extensively on US-Canadian relations. He thinks the Conservatives will place more emphasis than would the Liberals on business. ''I think the election of Mr. Mulroney would signal more of a move to further economic integration between the two countries. There would probably be greater responsiveness to the concerns of the investment, banking, and trade communities in both countries.''

Carroll cautions that the Tories, if elected, would not have a completely free hand because of political realities at home. ''Obviously Mr. Mulroney would have to be responsive to the very legitimate desire of many Canadians with stronger nationalistic feelings.''