Legal immigration has been a sacred cow since the 1965 immigration law. No mainstream pol has ever suggested lowering the annual intake quotas.You're quite right that it has been conventional wisdom in politics to speak highly of legal immigration. In the case of Republican politicians, vocally supporting legal immigration is the shield they hold up to fend off attacks from Democrats who charge racism is at the root of GOP opposition to illegal immigration. Gov. Walker's current stance, if sincere and not an election ploy, is a bold departure. We'll have to wait to see the net effect on his candidacy.
It's a risky stance, due to big donor preferences and press flak, but it could be an election winner if he gets the nomination. Middle of the road voters who usually vote Democrat but like his position might either vote for him or sit the election out. Ditto for any traditional Democratic constituencies who feel they're being squeezed out of the job market.