I'd call it the presidential victory mandate. I guess he shouldn't actually do what he promised as the victory wasn't large enough? "Damned if you do, damned if you don't."
Also as you were corrected there is constant goalpost moving in terms of whom exactly should be deported and how.
If you’re really interested in public opinion people don’t support ICE and especially how do they do what they do.
What people call a mandate has been when a position actually has been popular enough that it could be acted on quickly without opposition. The very fact that there is so much opposition to this, that the polling is so bad, that the protests are so widespread, that the negative feedback is so frequent even within the Republican party quite explicitly shows that there is no mandate here.
But again: If Trump's victory was enough to secure a mandate, why didn't a similar EV victory and significantly larger popular vote victory secure one for Biden? Why would a mandate based on deporting violent criminals apply to people without convictions? To people showing up at their court cases as they go through a legal process where they did follow all the rules?