Image, George Hodan
This section assigns electoral protection of the right to vote to both state and federal governments. They can watch each other's conduct. Both of these detail existing Constitutional powers, and they balance each other (for more on this amendment’s power balances, see The Balance of Powers in Emergencies, Judicial Review, The Constitution and Politics, Balancing Access Inside Congress, Enforcing Political Integrity in Congress).
It should be noted that the United Nations is not private but intergovernmental: if we ever needed external monitors or auditors, it could provide them (see Defining footnote).
In 2004, the United States volunteered to have some elections observed by an international organization called the OSCE. But because it has no legal authority, the OSCE's observations are for research purposes only. And because it's voluntary, eight states have already passed laws against it.
Legally binding, mandatory external monitoring or auditing
would still require UN involvement.