That’s not a problem. Anyone can campaign at any time. That’s totally legal and fine.
What’s absolutely absurd is the level of support he still gets from Republicans! He owns them.
If some former Democrat president did this stuff the support from Democrats for that person would be near zero. No one would consider such a candidacy even remotely within the realm of possibility.
Republicans just don’t give a shit about corruption. Some will see this statement and say, “no, they care about it when it’s not one of their people” but that’s 100% the same thing. Corruption is all about picking winners and losers based on what your side thinks of them.