Sen. Rand Paul, R-Kentucky
PHOTO: AP Photo/LM Otero
Sen. Rand Paul, R-Kentucky
(CNN) —  

Sen. Rand Paul ripped into President Barack Obama’s upcoming announcement on immigration action, saying Thursday the President has declared a “constitutional war” and warned that the fight against his executive order “won’t go away lightly.”

“He’s fundamentally misunderstanding our constitutional republic and really collapsing this separation of powers that’s so important,” the Kentucky Republican said in an interview with John Gibson on Fox News Radio.

Paul, who’s seriously considering a presidential bid, joins a chorus of mostly Republican voices denouncing the President ahead of his prime time appearance Thursday, when he’s expected to lay out plans that would allow up to 5 million illegal immigrants to stay in the United States.

Obama’s immigration speech: What to watch for

The President had repeatedly warned this year that if Congress failed to pass comprehensive immigration reform, he would use the executive branch to move on his own.

“This isn’t about immigration reform. This is about his usurping of power that the Constitution didn’t give him,” Paul said, arguing that Obama is “acting like a king.”

“To me this is a constitutional war that he has declared,” he continued.

The first-term senator vowed that Republicans will act when they hold more leverage in January under the new Senate majority. Using spending bills, he said they can “direct his behavior” and “restrain his lawlessness.”

The legal battle over Obama’s immigration plan

He also set the stage for a court battle, urging House Republicans to immediately pass a resolution condemning the action. The measure, he argued, could be used as the basis of a legal challenge.

“It may not do anything directly, but it may set up the precedent or the information the court needs to say that what he’s doing is in defiance,” he said.