That is what Stephane Dion did and he flopped the 2008 election. Trudeau wisely kept saying pollution pricing instead of carbon tax during the 2015 campaign and then introduced what in effect is a law like the Canada Health Act - the National Floor Price on Carbon. I think Pallister is overestimating his legal advice - pretty sure Trudeau has legal measures he can use to force Manitoba to comply like hike the federal gas tax in Manitoba or add a Carbon surcharge on top of w.e tax Pallister imposes to comply with the Floor Price mandate.