Durham, N.H. — The United States has long taken its northern neighbor for granted. To the man on the street, Canada is often perceived as a friendly country within the US sphere of influence. It shares the world's longest unguarded frontier, sports a maple leaf on its flag, and produces fine hockey players. Canada has even been referred to as the 51st state.
But unless that perception changes, and unless differences over political, economic, and environmental issues are resolved, the US-Canada bond may be seriously damaged, according to analysts.
The relationship between the United States and Canada is in such a state of deterioration that there may be ''resulting harm to the interest of both peoples if such deterioration is allowed to continue,'' warns Kenneth M. Curtis, former governor of Maine and ambassador to Canada under President Carter, and John E. Carrol, a specialist in Canadian-US environmental relations at the University of New Hampshire.
The solution, Dr. Carroll and Mr. Curtis suggest, is a new Canada policy in Washington that would carefully look at US-Canadian conflicts and deal with them before they become major issues.
In a book to be released tomorrow - ''Canadian-American Relations: The Promise and the Challenge'' - Carroll and Curtis argue that despite the efforts of recent Canadian outreach programs, Americans ''still know very little fact and have even less insight into this highly sophisticated and globally conscious society north of the border. While some isolationists might argue that ignorance is bliss, this state of affairs is undesirable and insulting at best, costly or even dangerous at worst. It is in the national interests of both peoples that the American people develop a greater and more insightful knowledge of their fellow North Americans.''
In economic terms, few US citizens realize Canada and the US are each other's largest and most important trading partners. Canadian exports to the US in the early 1980s reached $45 billion. US exports to Canada were worth $47 billion. Such is the interdependence that Canada invariably follows every fluctuation in the American economy - except that hard times are often harder in Canada. And recent US calls for protectionist trade policies curry little favor in Canada, experts say.
Politically, even fewer Americans acknowledge that the close relations and economic integration enjoyed by the two countries between World War II and the early 1960s have not carried through into the '80s.
Canada did not support US involvement in Vietnam, and more recently, has had little sympathy for US foreign policy in Latin America. The Canadian government is equally displeased with the lack of US consultation about, and subsequent military intervention in, Grenada.
There is also little US tolerance - especially under Reagan - for the social democracy of Canada's Liberal governments during the past decade, Dr. Carroll says.
At the same time, increasingly serious environmental conflicts, such as acid rain, use of the Great Lakes, fishing rights, and energy use, have strained relations.
In an interview here, Dr. Carroll says Canadians must face up to a dilemma: ''Do they want a high standard of living and at the same time surrender economic and polictical decisionmaking to Wall Street and Washington; or do they want to make their own decisions and take less. The question is: Just how much willingness is there to suffer economic sacrifice?''
Carroll and Curtis propose a new Canada policy that would include:
* A high-priority presidential commitment to US-Canada relations.
* A deputy assistant secretary of state for Canadian affairs. (The Reagan administration has appointed James M. Medas to the Canadian affairs post.)
* The creation of formal and informal institutions or commissions, governed by rules of procedure to deal with the day-to-day problems.
* Encouragement and support to subnational government to develop Canadian expertise.