Fair point. I am not asking for "one more level". The design already assumes parity. Each branch has a lever to counter the others - some faster than others, and none sits above the rest. Congress has laws, the purse (TBD), oversight, and impeachment. Courts can halt and review. The executive has veto, charging choices, and pardons that reach only federal crimes and never impeachment or civil liability. The whole point of this is when one of three branches misfire, the others answer through state cases, impeachment, elections, or new statutes.

The threat is lateral, not vertical. The system works by equal tension, not hierarchy.

Can impeachment be used to invalidate a federal pardon? My understanding is the President could certainly commit a crime by granting certain pardons, and could be removed by impeachment for that, but the pardon would still take effect over any actions of any branch of government and (arguably more importantly) a jury (which is one of few genuinely democratic institutions).

Short answer is no. Impeachment can remove a president for abusing the pardon power and deter future use, but it does not undo what would otherwise be a valid federal pardon. Once issued and accepted, a pardon binds federal courts and prosecutors. A jury cannot convict on a pardoned federal charge because the case is dismissed or the conviction is vacated.

The "check" to this power itself is also the reach: A pardon does not reach state crimes or civil liability. And the "check" to the person with the power of a federal pardon (the President) is political removal and later criminal charges like bribery - subject to Article 1, Section 3, Clause 7 of the Constitution [1]:

"Judgment in Cases of Impeachment shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States: but the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law."

1 - https://constitution.congress.gov/browse/article-1/section-3...