This may come to you as a surprise but there a quite a number of senior Republicans who agree with what Obama did to Russia. Some felt he did not go far enough e.g McCain . Yes it is true that Obama was aware that Russia has been hacking US secret government documents. The reason for his actions was a tipping point because Putin was actively trying to interfere with the election.

Yes Russia has been doing this with other countries but does that mean the US should do nothing?  Trump is boxed in and he cannot overturn the decision even if he wanted to.