President Barack Obama's planned executive action on immigration is a "big mistake" that will make working with the GOP on other issues more difficult, Sen. John Thune told CNBC on Thursday.
"The president ought to work with the Congress, instead of against the Congress," the chairman of the Senate Republican Conference said in a "Squawk Box" interview.
The president on Thursday night is scheduled to announce plans that would lay the groundwork to protect millions of undocumented immigrants from deportation.
Questioning the legality and constitutionality of Obama's approach, Thune said Republicans also "disagree in a very aggressive way" with the substance of how the White House wants to handle immigration policy.
The South Dakota senator said the problems that lead to illegal immigration, such as porous borders, need to be solved first. Then "there is probably a way in which we can address people who are here in this country illegally."