A useful way to hold this phrase: picture a group standing up in public and saying two things at once — we know what is right, and there are more of us than of you. The second claim is what gives the first one its force. Nobody has counted, and that is exactly the point: the numbers are asserted, not measured. Every use of the phrase, from the organisation that took it as a name to the cool way a newspaper uses it today, is standing on that double claim.
From the word’s history
The name was coined by the conservative activist Paul Weyrich in 1979, and became widely known through the televangelist Jerry Falwell. So the phrase started as a claim made on purpose, and the lower-case general use kept both halves of it: the moral standing, and the assertion of numbers.
根據資料，這個名字是一九七九年由保守派人士 Paul Weyrich 造出來的，後來透過電視佈道家 Jerry Falwell 廣為人知。也就是說，這個說法一開始就是刻意提出的主張。後來小寫的一般用法把兩半都留了下來：道德上的位置，加上人數上的宣稱。
Wiktionary, "moral majority"