He decided to agree with them and back them. It may be that he agreed with them, but being a senator, we will never know his true opinion. He has "claimed" to have the opinion of the critics of reforms. If he aligns with critics of reforms, he claims he does not want reforms. He and they either want things to stay the same, or he and they want different reforms than these.

We have this structure "align oneself with sb/st" that means like "ally oneself with sb/st" also "combine in Alliance". About your xample: "The senator aligned himself with the critics of the proposed reforms"=>we can understand that the senator and the critics of the proposed reforms had colluded together to make it[(the reforms which the senator had proposed)] come true(or will be done). (ps: it's because of political purpose of politicians,well...we can see that in the fact now). Do you understand?