Obama himself makes the greatest argument for his own impeachment:

“The President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation.” -Barack Obama Dec 20, 2007

“In instances of self-defense, the President would be within his constitutional authority to act before advising Congress or seeking its consent. History has shown us time and again, however, that military action is most successful when it is authorized and supported by the Legislative branch.” Barack Obama Dec 20, 2007

In a followup question in its December 2007 interview, the Boston Globe asked Obama if the Constitution gave the president the power to disregard a congressional statute putting some type of limit on the way troops could be deployed. Here, too, Obama deferred to the constitutional authority of Congress.

“No, the President does not have that power,” Obama told the paper. “To date, several Congresses have imposed limitations on the number of US troops deployed in a given situation. As President, I will not assert a constitutional authority to deploy troops in a manner contrary to an express limit imposed by Congress and adopted into law.”

Source:
http://cnsnews.com/news/article/obam...ave-power-unde

Dennis Kucinch (D-OH) thinks so too:

"Here it is, and I'm gonna read this and then I'll tell you who said it. "The president does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation." Now, that was Barack Obama who said that on December the 20th, 2007. We've got to be very sure here that we follow the Constitution, and President Obama didn't do that."

And so does ultra-leftist Ralph Nader:

"Why don't we say what's on the minds of many legal experts, that the Obama administration is committing war crimes -- and if Bush should have been impeached, Obama should be impeached."

I give Nader and Kucinch props for being consistant here. Of course you do have to marvel at the audacity of the community organizer though.