"Politics stops at water’s edge" -- meaning

I read the following phrase in the topic of foreign policies of a country,

"It stops at water's edge."

What does "politics stops at water’s edge" mean?

Wikipedia mentions "politics stops at the water's edge" as "forging bipartisan support for [Truman's] foreign policy" (by a Republican senator supporting a Democratic president).

Vandenberrg official biography also mentions "we must stop “partisan politics at the water's edge," he cooperated with the Truman administration in forging bipartisan support for the Truman Doctrine, the Marshall Plan, and NATO"

Google will provide many more examples of the phrase, some with explanation, many without (because understanding of the context is assumed).