Urgent: Should U.S. Strike Syria? Vote Here
The former presidential candidate and best-selling author also says he prefers "the devil we know" in Syria — Bashar Assad — to the al-Qaida elements he asserts are leading the rebellion against his regime.
Buchanan has been a senior adviser to three presidents, a two-time candidate for the Republican presidential nomination, and the presidential nominee of the Reform Party in 2000.
President Obama has signaled that he is considering a strike on Syria amid administration claims the Assad regime has used chemical weapons.
In an exclusive interview with Newsmax TV on Thursday, Buchanan says his chief concern about a potential strike is that "the president of the United States is threatening a war and planning a war he has no right to wage. The Congress of the United States alone has the power to authorize war or declare war and it has not done so.
"President Obama is usurping the authority of the Congress first and foremost, and he appears about to launch an unconstitutional and unnecessary war. So the president should be called to account by the Congress and told: no war without our approval. That's the way the Constitution works.
"The key figure is Speaker of the House John Boehner, who should call the House of Representatives back into session on Monday and instruct the president directly: Mr. President, you have no authority and no right to launch acts of war against Syria against whom we have not declared or authorized any war. We are calling on you not to engage in what would clearly be an impeachable act — starting a war against a country without the approval of the Congress when you are asked directly not to do so.
"If the president launched an unnecessary and unconstitutional war, striking a country against whom we have not declared war and has not attacked us, that is de facto an impeachable act that could lead to an open-ended war, the consequences of which we cannot even see."
The White House has talked about the moral justification for a strike. Asked if there also is a legal justification, Buchanan responds: "There's no constitutional justification right now in my judgment for a strike on Syria. The U.N. Security Council has not authorized a war, the Congress of the United States has not authorized a war.
"I do agree that the use of poison gas by the Syrian government — if it was President Assad who authorized it — is an obscene act which the international community and the Security Council should take up. But we don't know who ordered it; we don't know how it was delivered; we don't know if Assad knew about it; we don't know if Assad ordered it.
"But if he did, this is an issue that ought to be taken up by the international community and the Security Council, not the United States of America unilaterally, and certainly not the president of the United States, based on the flimsy evidence we have seen to date."
Obama declared unequivocally on Wednesday that the Syrian government was responsible for the chemical attacks on civilians. However, several U.S. officials are now using the phrase "not a slam dunk" to describe the intelligence picture.
Buchanan comments: "I would not understand or comprehend if Assad, no matter how bad a man he may be, would be so stupid as to order a chemical weapons attack on civilians in his own country when the immediate consequence might be that he would be at war with the United States.
"But what the United States should do is quite clear: Gather all the evidence through the U.N., gather all the evidence through our intelligence, take this to the Security Council the same way President Kennedy through Adlai Stevenson took the [evidence] during the Cuban Missile Crisis. We had our photographs, we showed the world what we had, we proved the missiles were in Cuba.
"That is the constitutional and legal way to do this. It is not to act in panic because John Kerry is shocked at the pictures he saw on YouTube."
Buchanan said Senate Majority Leader Harry Reid should call the Senate into session and "if he believes we should go to war, authorize it."
"That is what George H.W. Bush did before he drove the Iraqis out of Kuwait. That is what George W. Bush did. I was against that war on Iraq but the president won the authority from Congress so it was a constitutional and legitimate war no matter that I did not like it."
If Obama does attack Syria without approval, "it is a clear, unconstitutional, illegal act," Buchanan reiterated. "If the president did this, he would be a rogue president."
Buchanan says he disagrees with former U.S. Ambassador to the U.N. John Bolton's assertion that we should seek to take out Assad.
Urgent: Should U.S. Strike Syria? Vote Here
"Look who is on the other side of this war," he tells Newsmax. "We have al-Qaida elements that are murderous, that have tortured people, that have killed Christians, and they're the leading force in the elements that are fighting against Assad.
"Behind Assad we have the Iranians and Hezbollah and the Russians. It is not our war. Quite frankly, I would prefer the devil we know, which is Assad, to the devil we don't know, which is that crowd in the rebels who are torturing and killing people and engaging in atrocities of their own."
Buchanan also says the Republicans have "the power of the purse" and should block spending by those agencies that would implement Obamacare.
And regarding immigration reform, Buchanan doubts the GOP-controlled House will go along with the amnesty that Obama wants and the Senate has approved.
He adds: "I believe and hope that the House of Representatives will deny amnesty, deny legal rights to people, who've broken into our country and broken our laws."