If President Bush were to come to Canada before the next election and endorse a specific candidate or political party, I would consider this an honest comment. Obviously he is going to have preferences and we may as well know what they are up front. Canadians would not automatically rush out to vote for the candidate that he endorsed, nor would they avoid voting for that candidate en masse - they would simply take the American preference for what it is, the preference of a foreign head of state.
It would be interesting to see what affect it would have on the election. Canada has generally fared worse economically when our Prime Minister has been too friendly with the American President. Nixon disliked Trudeau and we did fine. Reagan got along well with Mulroney and the Canadian economy faltered badly. Bush disliked Chrétien and the Canadian economy roared along. As Bush is improving his relationship with Martin, our economy is slowing down again. Endorsement of a candidate by Bush might cost that candidate the election.
Canada is in a special position because of our geography. We share the least defendable border in the world. As well, we are America´s biggest trading partner in both directions by a wide margin. The United States cannot afford to disrupt trade with Canada without doing damage to its own economy. Even small disturbances in US-Canada trade cost Americans. The ongoing softwood lumber dispute has increased the cost of construction in the US and damaged the American building trades; the mad cow fuss has increased the cost of meat in American supermarkets and hurt the American meat packers (the majority of Canadian beef was processed by American companies). And we can´t forget that Canada is the largest supplier of hydrocarbon energy (oil, gas, and coal) to the US. No matter how the President feels, Canada and the United States will continue to have a close relationship.
We fear the American cultural influence much more than the American military. Hollywood, not the Pentagon, is on the forefront of the American invasion of Canada. Other countries that consider military invasion by the US might consider an endorsement by the American President to be a veiled threat. But it would probably have a fairly small impact on the elections in most countries. I doubt that industrialized countries feel that they are likely to be invaded by the US any time soon and the less industrialized countries don´t seem to hesitate to tweak Uncle Sam´s nose in rather more painful ways than voting for candidate that was not endorsed by the US.