The Crime or Rebellion clause should never have been passed. It violates three fundamental principles underlying the Constitution:
1. It isn't necessary, and neither rights nor privileges may be denied by Government without need. Denying the vote for crime or rebellion isn't necessary to any purpose listed in the Preamble. Nor does it improve any structure or function of government. It's just a punishment when we have no shortage of other punishments that perform the same social functions, like deterrence, without being denials. Most punishments are just abridgments, like the abridgment of freedom of movement.
2. Government may not abridge or deny the inalienable right to ordain and establish government. Even though the vote is first granted as a privilege, to have the vote is to have the inalienable right to continue to ordain and establish our government by exercising it. Allowing Government any power to deny voters this exercise not only gives Government power over an inalienable right, it gives it power over its own continued existence, nature and officers. We, the people, didn't institute a government that could then ordain and establish itself by deciding which voters could vote.
3. Government may not grant one person mastery (individually-exerciseable power) over another's inalienable rights. Government may not establish tyranny.
Amending this clause meets three compelling government objectives restoring the legal relationship between the people of the United States and our government.
"The United States shall guarantee to every State in this Union a Republican Form of Government, [...]"
Article IV, Section 2:
"The Citizens of each State shall be entitled to all Privileges and Immunities of Citizens in the several States."
A uniform right to vote for all citizens of legal age would finally cover the Guarantee clause's guarantee.
And the Equal Privileges and Immunities clause requires (at least) that moving from one state to another
won’t reduce a citizen’s fundamental rights, privileges or immunities.
According to these clauses, voter eligibility should have been standard from the beginning. But it wasn’t.
States disagreed on whether to require property ownership, some length of citizenship, male gender or white race for suffrage,
and on legal age (see The Representation Question if you want to compare this to the right to representation).
The Framers signed the Constitution without reaching a consensus, but instructed us by
the aspirational terms of these clauses to continue to seek it.
By 1868 some of these had been resolved and Amendment XIV was able to solidify more suffrage policy.
Just as it restates the main statement of the Apportionment clause to show that it’s not repealing it, Amendment XIV
restates the Equal Privileges and Immunities clause to show that it’s not repealing it.
In fact, it restores, extends and details it: we still have to eventually achieve consensus.
We didn't refuse the ballot to people convicted of crime or rebellion before the Civil War.
Had there never been a war, these permitted restrictions wouldn't have been added to the Constitution.
Amendment XIV was drafted at the end of the Civil War.
The people being punished for rebellion were the Southern officers.
After disallowing property requirements (see Self-Governance and the Income Inequality Question) and every
other once-disputed qualification for the vote except gender, Amendment XIV added two new ones.
Gradual change at the state level is always preferred but full suffrage has become a critical need (see Voting as a Civic Duty).
We rely on the ballot for a government that sets informed priorities and makes informed budgets.
And no true republic punishes citizens by removing their power to ordain and establish government.
It's high time we corrected the error.
Denying the vote to adult citizens isn't democratic, has never benefitted us and isn’t an affordable loss anymore.