SCOTUS will rule it is unlawful or unconstitutional as-applied for state officials, the Federal Executive by fiat, and Congress by a simple majority vote, to disqualify a candidate on the basis of the 14th Amendment §3 without a conviction in a federal court of law.
No person shall be a Senator or Representative in Congress, or elector of President and Vice President, or hold any office, civil or military, under the United States, or under any State, who, having previously taken an oath, as a member of Congress, or as an officer of the United States, or as a member of any State legislature, or as an executive or judicial officer of any State, to support the Constitution of the United States, shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof. But Congress may, by a vote of two-thirds of each House, remove such disability.
In the 14th Amendment §3, "shall have engaged in [...]" is a question of fact. Questions of fact are adjudicated in courts of law. For simple qualifications such as age, natural birth, and citizenship, the production of a a simple birth certificate will suffice. But none of these imply a question of criminal behavior. Treason, and impeachment followed by conviction, as discussed in Article 1 and 3, could imply a different procedure would be used. Which? We'll get to that.
The Congress shall have power to enforce, by appropriate legislation, the provisions of this article.
In the 14th Amendment §5, Congress—and Congress alone—is granted power to the enforce the 14th Amendment, and only by appropriate legislation. Not state officials, not the Federal Executive. And "legislation" implies passing a law, not a simple procedure like expulsion or judging the qualification of members (which only applies to the relevant house). Compare this with the language used to remove this disqualification in §3, where the authors chose to use "vote" rather than "legislation".
Now you could argue that, like the interstate commerce clause, states can fill in where Congress has chosen to be silent. However, if Congress is not silent then the Supremacy Clause under Article 6 takes effect. And history has spoken on this.
And be it further enacted, That if any person shall hereafter incite, set on foot, assist, or engage in any rebellion or insurrection against the authority of the United States, or the laws thereof, or shall give aid or comfort thereto, or shall engage in, or give aid and comfort to, any such existing rebellion or insurrection, and be convicted thereof, such person shall be punished by imprisonment for a period not exceeding ten years, or by a fine not exceeding ten thousand dollars, and by the liberation of all his slaves, if any he have; or by both of said punishments, at the discretion of the court.
The Confiscation Act of 1862 (
12 Stat. 589,
590) defines insurrection, rebellion, and giving aid or comfort as criminal offenses. While it doesn't define the elements—the Insurrection Act of 1807 may provide them—it does ground the idea and original public meaning that insurrection, rebellion, and giving aid or comfort are criminal offenses. It also doesn't list disqualification since the 14th Amendment was only passed four years later.
And be it further enacted, That whenever any person shall hold office, except as a member of Congress or of some State legislature, contrary to the provisions of the third section of the fourteenth article of amendment of the Constitution of the United States, it shall be the duty of the district attorney of the United States for the district in which such person shall hold office, as aforesaid, to proceed against such person, by writ of quo warranto, returnable to the circuit or district court of the United States in such district, and to prosecute the same to the removal of such person from office; and any writ of quo warranto so brought, as aforesaid, shall take precedence of all other cases on the docket of the court to which it is made returnable, and shall not be continued unless for cause proved to the satisfaction of the court.
In the Enforcement Act of 1870 §14 (
16 Stat. 140,
143;
Rev. Stat. §1786 (1874);
5 U.S. Code §14a (1925)), Congress uses the 14th Amendment §5 power to enforce 14th Amendment §3. Here, they specify a process by writ of quo warranto, wherein the DA gets the duty to prosecute and remove someone on the basis of 14th Amendment §3 disqualification in a court of law. Here, the elements of insurrection, rebellion, or giving aid or comfort must be proven, whatever they may be, but to which standard? Original public meaning would imply proof beyond reasonable doubt with a jury, i.e. a criminal trial, as insurrection, rebellion, and giving aid or comfort were and are considered criminal offenses. Moreover, the analog of proving manslaughter vs proving wrongful death implies the same standard as a criminal trial, and the people certainly knew of the difference as wrongful death has existed in some form or another even before the Fatal Accidents Act of 1846 in the UK.
But I digress for the point is moot, since the Enforcement Act of 1870 §14 was repealed by the Act of June 25 1948 (
62 Stat. 993), and replaced by the same with §2383 (
62 Stat. 808).
Whoever incites, sets on foot, assists, or engages in any rebellion or insurrection against the authority of the United States or the laws thereof, or gives aid or comfort thereto, shall be fined not more than §10,000 or imprisoned not more than ten years, or both; and shall be incapable of holding any office under the United States.
The writ of quo warranto was removed, and in its stead is a simple criminal statute. The section was later amended in 1994 to its current form, where the fine was changed.
Whoever incites, sets on foot, assists, or engages in any rebellion or insurrection against the authority of the United States or the laws thereof, or gives aid or comfort thereto, shall be fined under this title or imprisoned not more than ten years, or both; and shall be incapable of holding any office under the United States.
(
Source)
The point here is that Congress, by the power granted to it under 14th Amendment §5, has chosen to enforce 14th Amendment §3 with the criminal statute 18 U.S. Code §2383, thus Supremacy Clause is in effect, and any disqualification process that isn't a criminal trial of a charge under §2383 would therefore be unlawful or unconstitutional.