I agree with grandparent that Sessions' action will tend to increase pressure for federal decrim., and that might even be Sessions' (and/or President Trump's) goal.
What would the effect of Federal decriminalization be on the mid-terms? Don't you think it would help Republicans? Do you think they don't think so? Of course they think so.
I don't know if it will pass, but my guess is that a reasonable bill would decriminalize marijuana possession and trade, but would keep it illegal for interstate commerce (this would be a sop to those States that want to keep it criminalized). Sort of like the 21st Amendment, but as a statute. Such a compromise would help Republicans in States where legalization is popular and... would not hurt them in States where it is not. It would also help with the federalist types.
This should all be blindingly obvious. The only reason I wouldn't make a prediction is that Republicans sure know how to shoot their feet off.