What can the president legally alone do to regulate (prior and during) the presidential election?
<p>Given that the elections are covered by state law and administered by the local governments. I think legally there is very little (with express support of Congress) that can be done except seek the FBI to investigate possible actors who might be illegally acting. What can the president do alone?</p>

<h2>The appointment of electors is a matter for the States</h2>
<blockquote>
<p>Each State shall appoint, <strong>in such Manner as the Legislature thereof may direct</strong>, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector.</p>
</blockquote>
<h2>However, ...</h2>
<p>The States must still comply with the Federal Constitution and their own laws and practices of appointment must be in compliance with it. This is why Federal courts can be a venue for disputes about the appointment of electors.</p>
<p>The most common basis for such a dispute is the equal protection clause of the fourteenth amendment:</p>
<blockquote>
<p>All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.</p>
</blockquote>
<p>Or the fifteenth:</p>
<blockquote>
<p>The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude.</p>
<p>The Congress shall have power to enforce this article by appropriate legislation.</p>
</blockquote>
<p>Deciding if mail-in votes do or don’t count is squarely within the scope of the equal protection clause.</p>
<h2>As for the President and the Congress</h2>
<p>They have the power to ratify and legislate respectively, before an election, laws that give effect to the fourteenth and fifteenth amendments - the Civil Rights Act being the notable example.</p>

<h1>Nothing, not even <em>with</em> support from congress.</h1>
<p>The US constitution prescribes in <a href="https://constitution.congress.gov/browse/article-2/" rel="nofollow noreferrer">Article II, Section 1, Clause 2:</a></p>
<blockquote>
<p>Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector.</p>
</blockquote>
<p>Emphasis is the 2nd word and the method prescribed after that: <em>Each State, <strong>in such a manner as the legislature [of the state]</strong> may direct</em>. That is, the constitution prescribes that the choice of electors is a matter of the states and, as a result, never federal. The federal government can't change anything in that - the clause <em>explicitly</em> preempts congress from making the choice of electors a federal matter. It is long standing, that state legislatures have &quot;<a href="http://cdn.loc.gov/service/ll/usrep/usrep146/usrep146001/usrep146001.pdf" rel="nofollow noreferrer">the broadest power of determination&quot;</a> in <a href="https://constitution.congress.gov/browse/essay/artII-S1-C2-1-2-1/ALDE_00001121/" rel="nofollow noreferrer">deciding their process</a> how to choose electors. They may not however violate things like equal protection in setting up their decision-making process, and there are a very few federal legislation that applies - like voting age or suffrage.</p>
<p>If Hawaii should decide to make only descendants of the King of Hawaii possible electors and put that into their body of law in the proper way (presuming such a rule can hold up under Hawaii's constitution), that would have to be accepted by the federal government, unless they can show that this would break something in the US constitution.</p>
<p>If Nevada decides that they choose electors by doing a state poker KO tournament, at which end the last 6 standing are the electors, that rule can't be challenged in SCOTUS in the first instance, but the Nevada Supreme Court gets the first shot to decide on the constitutionality under the Nevada constitution before SCOTUS might weigh in and try to overturn on the US constitution.</p>

president elections