Longstanding Supreme Court precedent
indicates that only Congress can impose conditions on grants given to states and localities, and that those conditions must be “unambiguously” stated in the text of the law “so that the States can knowingly decide whether or not to accept those funds….”
Should the administration manage to get away with [its efforts to impose new grant conditions], it will set a dangerous precedent that goes far beyond… the specific issue of sanctuary cities. If the president can unilaterally add new conditions to one federal grant program, he can do the same thing with others. This would give presidents a massive club to coerce state and local governments on a wide range of issues….
Some conservatives may cheer when the current administration uses this tool against sanctuary cities. But they are likely to regret their enthusiasm if a liberal Democratic president uses the same tactic to force states to increase gun control, adopt a “common core” curriculum, or pursue liberal policies on transgender bathroom accommodations.
Allowing the executive to impose its own after-the-fact grant conditions also threatens the separation of powers. It goes a long way towards taking control over spending away from Congress and transferring it to the president. This, of course, violates the text of Article I of the Constitution, which clearly gives the power of the purse to the legislature, not the executive.
As conservatives often pointed out during the Obama Administration, the modern executive has already appropriated far too much power that more properly belongs to Congress or the states. It is dangerous to let it seize even more.