Why is Obama sliding?

Dave Lindorff has an article over at Counterpunch (here) that pretty much hits the nail on the head:

But how can I or any progressive vote for a presidential candidate who goes
from opposing a war to saying he not only supports the idea of keeping troops in
Iraq for another five years—the length of the entire WWII!—but who further says
he won’t rule out attacking Iran, even if that country poses no imminent threat
to the US, simply because it develops nuclear weapons—the same weapons that our
putative friends, Pakistan and India, have? How can I vote for a candidate who
wants to expand the military (by 65,000 troops) instead of shrinking this huge,
bloodsucking parasite of an organization which is costing as much as the rest of
the world spends on its armies?

How can I or any progressive vote for a presidential candidate who
cannot state categorically that he will defend the Constitution by reversing all
of President Bush’s abuses of power and who will not promise to prosecute the
president and members of his administration for any crimes committed while in
office?