If true...it will do nothing of the sort.
For example, Joe Biden and the Democrat leadership are free, even today, to categorically state that "The Surge was a failure".
The MSM reports the assertion...and totally ignores any evidence to the contrary. Thus, in time, a bald lie becomes "conventional wisdom".
If there is evidence that Saddam shipped his WMDs to Syria, the MSM will simply ignore it.
You forget there will be campaign where a vice presidential candidate or other surrogate can directly accuse the leadership and the rat candidate of gross incompetance while pointing to not only the new info but reams and reams of ignored info.
The msm will have no choice but to report the story.