Let us suppose that a sufficiently large majority of the US Congress, with the agreement of the President, decided that the death penalty should be abolished throughout the nation, both at the federal and state level. How could they make it happen?
Of course, they could abolish the federal death penalty with an ordinary act of legislation, and the President could pardon or commute the sentences of all federal prisoners on death row. But 31 states also provide for the death penalty, and in principle, Congress does not have the power to change their laws (Tenth Amendment). Let us suppose that those 31 states (and their legislatures, governors, courts, etc) are intent on preserving the death penalty, so that a Constitutional amendment to abolish it would not be ratified by 3/4 of state legislatures as required.
The Supreme Court could effectively abolish it by ruling that it constitutes cruel and unusual punishment under the Eighth Amendment (which also applies to the states via the incorporation doctrine and the Fourteenth Amendment). But let's suppose the Judicial Branch is not on board.
So how can Congress and the federal government proceed, legally?
They could agree to a "litmus test" policy, to appoint / confirm only Supreme Court justices who they could trust to vote for abolition, as above. But this would not be effective until sufficiently many vacancies had opened on the Court and been filled, which could take many years. Moreover, the justices thus appointed could change their minds.
The President's power of clemency only extends to federal crimes, right? He/she has no power to pardon crimes under state law, nor to order state prisoner reprieved. So that doesn't help.
They could attempt to coerce the recalcitrant states, by denying them federal funding of one sort or another until they change their laws. This presumably would work, but would depend on how stubborn the holdout states were, and how much chaos Congress was willing to cause to the public in the meantime (e.g. no roads getting fixed, no Medicaid, etc).
They could make it a federal crime for any state official to perform or authorize an execution. But would this pass Tenth Amendment scrutiny?
They could pass a Constitutional amendment banning the death penalty, and specify that it should be ratified by state ratifying conventions, as was done for the Twenty-first Amendment, thus bypassing the state legislatures. However, if the death penalty had popular support in more than 1/4 of the states, the conventions might not ratify the amendment either.
Are there any problems or possibilities with these options that I've missed? Are there other options that might be feasible?