Easy - they can just threaten to halt the block-payments the state government relies on.
So - does this mean that the Constitutional limitation of the Fed over the States means nothing because all the States need fed monies? I can picture a universe where the powers are balanced out as the Constitution requires and where the States, refusing to go along with all the expensive and regulatory dictates, get along just fine. Might take a while for the balance to be restored, but eventually it would become stable and we'd all be better off. Unless of course we were willing to leave dictators in charge of pogroms that the Feds are supposed to benignly provide.