Yahoo News: If President Barack Obama chooses to unilaterally launch a military attack against Syria in retaliation for the government’s alleged use of chemical weapons against civilians last week, he is certain to face criticism that he’s overstepping his executive authority.

The president has already run up against resistance from some members of Congress, who argue that under the 1973 War Powers Resolution and the U.S. Constitution he must seek the body’s full approval before taking military action against the country.

The disagreement is part of a larger and thorny constitutional and legal argument over how far Congress can go to check the chief executive’s war powers and what types of military actions constitute war.

Rep. Justin Amash, R-Mich., has said it would be “unquestionably unconstitutional” for Obama to bomb the country without Congress’ approval, and he has authored legislation to withhold funds from the effort. Democratic Sen. Tim Kaine of Virginia also has suggested the president might be on shaky legal ground if he doesn’t get a congressional OK. More than 100 members of Congress signed a letter to the president warning him to seek their approval before attacking another country.

Interestingly, Obama himself made a similar argument while on the campaign trail six years ago. He told the Boston Globe in 2007 that no president can use military force absent an “actual or imminent threat to the nation” without first getting Congress’ approval. (Vice President Joe Biden, for his part, vowed to impeach President George W. Bush in 2007 if he bombed Iran without first getting approval from Congress.)