Washington (CNN) Sen. Rand Paul declared the war against ISIS "illegal" in a new opinion piece Monday, urging conservatives to speak out against President Barack Obama's decision not to seek congressional authorization for military action against the terrorist group.

"This war is now illegal. It must be declared and made valid, or it must be ended," the Kentucky Republican wrote in an op-ed published Monday by The Daily Beast.

Paul cites the War Powers Resolution of 1973 in stressing his long-held position that all military action must be approved by Congress. The exception is when a situation is considered urgent. In that case, a president has 60 days to get authorization after military action. If there's no approval from Congress, the president has 30 days to end the mission.

While Paul said he doesn't think the War Powers Resolution specifically applies to the ISIS situation -- because the U.S. had not been attacked -- he notes that even if it did, the president's time would be up.

"Taking military action against ISIS is justified. The president acting without Congress is not," he wrote, reiterating a stance he's held since September. This fall Paul has described the airstrikes against Syria as appropriate action but said Obama's method for doing so was "unconstitutional."

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The op-ed comes as Paul, who's seen as a likely presidential candidate in 2016, has been trying to shed his isolationist image yet maintain his libertarian-leaning roots.

In the new op-ed, he specifically reached out to conservatives, saying they "they should end their conspicuous silence about the president's usurpation of Congress' sole authority to declare war."

He suggested conservatives are being hypocritical in their criticism of Obama by lambasting him for acting alone through executive actions but staying quiet when he authorizes military action without approval.

"Conservatives who blast the president for ignoring the separation of powers on immigration display a fatal inconsistency by embracing unlimited war-making powers," he wrote.

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His piece comes as a report from Politico lists new details about his all-but-certain presidential campaign. The report says he would likely headquarter his campaign in Louisville, Kentucky, and would move forward with a 2016 re-election bid for the Senate on top of a presidential run.

Paul, who's attempting to sharpen his foreign policy brand, also raises questions about Hillary Clinton's physical stamina as well as her record as secretary of state and her involvement in Libya.