以～为: take...as...
In 为首， 首 means head or leader, while 为 means as.
For example, 以瘦为美：take thin as beauty; 以他为首： take him as leader;
So, 以安倍的自民党为首: take 安倍的自民党 as leader. 以安倍的自民党为首的 is attributive to modify 联盟. 安倍的自民党 is the head.
dan
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