It could be that this is massive, and has little or nothing to do with the US presidential election. They're going to try to force their way into social by grafting social media posts onto the one thing basically everyone uses - the search page.
Or it could be a desire to provide maximally relevant search results. Sometimes a cigar is just a cigar, Dr. Freud.
Not-a-disclaimer: I'm spending the year at Google (as a visiting scientist; I don't own stock and have no financial interest in its success). You might be surprised by how strong the desire is to do the right thing by users and make it easy for them to find what they're looking for. There's not always an ulterior motive -- happy searchers made Google what it is today.
This doesn't seem like such a bad idea. I don't care about most of the stuff on Twitter. But, if I'm searching for something and a Tweet-like "post" exists that is relevant to what I'm searching for, I might like that.