I agree that Section 3 may require Congressional action as to senators and representatives, but not for Presidential electors. Article II, Section 1 delegates to the state legislatures the power to decide how the state will "appoint" electors. It is entirely possible, therefore, that in some states, legislators pledged to a felon, say, may not be appointed, and in other states, such electors could be appointed. The decision by the state not to allow its electors to support a particular candidate for some reason not offensive to the Federal Constitution has no bearing on what other states can or must do. (I'm not making the "Independent State Legislature" argument here; let's suppose that all of the ordinary processes, checks and balances of state government apply.)

Article II, Section 1 distinguishes the President from representatives and senators vis a vis U.S. Term Limits Inc., v. Thornton, which is all about the selection of legislators. Article I very clearly takes from the states the power to decide who will serve in the federal legislature. But Article II just as clearly gives to the states the power to decide who will speak for each of them in presidential elections. No one runs for president. People pledged to candidates run for elector. The name on the ballot is just a metonym for electors pledged to that candidate. If the appropriate state official, acting under state law (as opposed to a judge applying the Fourteenth Amendment directly) determines that the state cannot appoint as elector anyone pledged to someone who, the state official finds, is ineligible for the Presidency under any Federal constitutional provision, barring that candidate's name from the ballot is within that officer's power.

Thornton held that states cannot impose requirements for service in the Federal legislature. But I see no reason a state could not, if its laws so provide, refuse to appoint anyone as elector who is not pledged to a candidate over the age of fifty. Likewise, a state could adopt by statute, if its constitution allows, its own version of Section 3, applicable to the President and without the annoying prior oath requirement. I am not saying that any state has done such a thing or that, under the law of any state, including Colorado or Maine, any official has the authority to exclude Trump from the state ballot. What I am saying is that state law governs who is an elector, which means that, to the extent a state dots all the i's and crosses all the t's, states can reach different conclusions without creating a constitutional crisis, and congressional action is not required to make Section 3 operative.

Of course, no state has legislated a rule that Section 3 applies to the Presidency, so a finding by SCOTUS that it does not apply would, under state law, bar state officials from disqualifying electors merely because they support someone who is otherwise described by Section 3. (I believe that Section 3 does apply to the Presidency, but that's for another day.)