Is Sen. McCain still competent enough to hold office?

It was another hearing with a lack of the expected fireworks today, but one thing that did strike me was just how confused Senator McCain came across during his questioning. He seemed to be conflating the Clinton e-mail investigation, and Comey's decision not to recommend charges, with the current Russia investigation. It was a very baffling line of questions, that was peppered with McCain misspeaking (e.g. saying President Comey), and left the viewers, Comey and even Marco Rubio a little perplexed.

I don't ask this because of any personal dislike for Sen. McCain, I think he's served the country for a long time in the way he thought was best, but he is 80 years old and this was a troubling event to watch. I have to wonder if he is becoming senile in his old age, and whether or not he is currently is capable of performing his duties as a Senator.

Here is the video of his portion:

http://www.cnn.com/videos/politics/2017/06/08/john-mccain-questions-james-comey-sot.cnn

All in all it was a very strange sequence of questions, that somehow tried to link the Clinton e-mails into the Russian collusion investigation. I can't make much sense of it other than his cognition was impaired. Whether its due to his age and this is just who he is now, or if it was the result of extenuating circumstances, I don't know.