Left Cover is a term that describes how the perceived left wing of mainstream politics spin an issue in such a way that the perceived right wing of mainstream politics can carry out their agenda anyway.
In other words the left covers for the right. Like a child covering for their friend when they do something wrong.
However it is a term that is generally used by the apolitical who see the the left and right as a false and simplistic paradigm, where both sides are corrupt and often seek the same agenda by different routes.
Left cover can be carried out unintentionally or as a direct conspiracy between left and right, although it is never clear to what degree certain individuals are conscious of their actions.
Obama preached change from George Bush, but has given the left cover for the same eradication of the US constitution and further wars of aggression.
The right may be after oil reserves, infrastructure contracts, and regime change. This is enabled by left-wing commentators who claim we need to prevent a human rights disaster at the hands of Gaddafi. While the left gain support from those concerned with human rights, they give cover for the right to carry out what might be seen as an imperialist agenda.
It must be noted that this definition of left cover is simplified and theoretical. In the real world there are many different factors that go in to political decisions. Furthermore something that might appear to be left cover could be entirely coincidental.
The term is closely related to “gatekeeper”, somebody that will act in opposition to part of a political agenda, but not to the point of disrupting the full agenda. Michael Moore is often considered a left wing gatekeeper on topics such as 9/11 Skepticism. He criticized the Bush administration and revealed some facts about the attack that painted Bush in a bad light, but didn’t go as far as revealing evidence that suggested elements within the American establishment were complicit, such as the granting of the terrorists’ visas. Thus he is not letting people see beyond the gate and is playing in to the left right paradigm.
People who adhere to left wing politics in its purest form do not appreciate the term left cover. In their mind anybody using left cover is not left wing and are painting them in a bad light. I.e. Obama is not left wing, he’s just using left cover to gain support.