the use of 'which'

[learnerr]

Hope whoever sees this helps me out

This is quoted from New York Times,
the article titled " Romney Appears the Ohio Winner ; Santorum Strong "

" But all eyes were on Ohio, which The Associated Press called for Mr.Romney early Wednesday morning, capping a turbulent night in which the result see-sawed both ways within a very tight range."

I can understand the general concept of this line ; Ohio was the key state in this nomination race so it took much of attention. And the result made Mr. Romney a winner but he could only pointed to the squeaker.

Q : The problem I have is , the use of which in front of Ohio. What does which refer to? If it refers to Ohio, whose objective is it?
I believe it would be correct to write Where rather than Which.

In addition what does call for mean in that line?

[Tdol]

They're thinking of it as an electoral contest and if you used where, you would be saying that the Associated Press were in Ohio, which is not the case. Which refers to the contest in Ohio.

[Tdol]

PS Call means declare someone to be the winner before the results are all in but when it is clear who will win.