[EL] Rudimentary 501(c)(4) question

[Marty Lederman]

I probably should know this -- so apologies in advance if there's a
well-known answer -- but I've never really thought about it, and it's an
obvious question in light of recent events:

When did the IRS decide that “[a]n organization is operated
*exclusively*for the
promotion of social welfare [the statutory standard] if it is *primarily*
engaged in promoting in some way the common good and general welfare of the
people of the community”?  26 CFR 1.501(c)(4)-1(a)(2)(i)

And why?  Since Congress chose the adverb "exclusively," how and whey did
the agency and everyone else settle on "primarily"?
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