"<to be> in the tank for <someone>"

[Mark Spahn]

Gentlemen (and ladies):

Someone told me today that this would be an excellent
place to ask about a certain phrase that seems to be new
in 2005 and that I cannot find in any dictionary.


The phrase "<to be> in the tank for <someone>" means
to be a partisan in favor of a given person, or at least that
is what I infer from uses of this phrase, such as
"the New York Times is in the tank for Hillary".
What, precisely, does this phrase mean?
Is it pejorative to be in the tank for (never "with") someone?
Is this phrase used only, or predominantly, in the United States?
What is its origin?  Did it come from sports, or
did a bit of chemical engineering jargon
make its way into the mainstream in 2005?
I did not find the answer via Google,
but that just means I'm an unskillful googler.

-- Mark Spahn  (West Seneca, NY)