The Preamble’s implied bright-line rule: Public conduct must be free from influence by private transaction, whatever its intent.
While the Journals clause and the Statements and Accounts clause are the only places where the Constitution addresses self-governance information directly, the Preamble implies a bright-line rule, a check on private power.
The Preamble (proposed for restoration, see Restoring the Preamble):
"We the People of the United States,
in Order to form a more perfect Union,
insure domestic Tranquillity,
provide for the common defence,
promote the general Welfare,
and secure the Blessings of Liberty to ourselves and our Posterity,
do ordain and establish this Constitution for the United States of America."
Elections are public self-governance conduct, the way we ordain and establish government. Voting is public conduct. Political speech, running for office, or associating for a march or rally are public conduct. Their free and aggregate nature can be seen. Private transaction is private conduct.
Private conduct may not interfere with the public interest,
especially a vital public interest like self-governance(see Public vs Private Conduct).
The Preamble’s implied bright-line rule is neither vague nor difficult to enforce:
Public conduct must be free from influence by private transaction,
whatever its intent.
The public conduct of ordaining and establishing United States government by election, and the public conduct of governing the direction of the exercise of United States offices, must be solely financed by the United States.
Amendment XXVIII, (proposed), Section 1:
"The people of the United States
will ordain and establish United States government under this Constitution,
and the United States will fund and certify self-governance during a period set by Congress,
uniform throughout the States,
preceding any vote in an election, appointment or Act."
Amendment XXVIII, (proposed), Section 5:
“The United States will fund the direction of exercise of its offices.”
Implied rules are real. The Supreme Court’s authority to interpret the Constitution is implied.
So, to restore the Preamble’s full legal force (see Restoring the Preamble) would technically invalidate private campaign financing - but expressing and detailing the rule will work better than implying it.