以～为: take...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.