At the end of the Christmas Invasion episode, after Harriet Jones orders to shoot down the alien spaceship, the Doctor, visibly mad, tells her he could take her down with a single word, or rather six.
The exchange went like this:
DOCTOR: Don't challenge me, Harriet Jones, because I'm a completely new man. I could bring down your Government with a single word.
HARRIET: You're the most remarkable man I've ever met, but I don't think you're quite capable of that.
DOCTOR: No, you're right. Not a single word, just six.
HARRIET: I don't think so.
DOCTOR: Six words.
HARRIET: Stop it!
DOCTOR: Six.
(The Doctor goes over to Alex and whispers in his ear.)
DOCTOR: Don't you think she looks tired?
(The Doctor, Rose, Mickey and Jackie leave.)
HARRIET: What did he say?
ALEX: Oh, well, nothing, really.
HARRIET: What did he say?
ALEX: Nothing. I don't know.
HARRIET: Doctor! Doctor, what did you? What was that? What did he say? What did you say, Doctor? Doctor! I'm sorry.
After that, we see glimpses of BBC news reporting a possible vote of no confidence over Jones, and an interview with her in which she insists she's fit and can do the job.
I never understood what actually happen. Did the Doctor hypnotize Alex, the assistant? How did they go from those six words spoken to him, to the Parliament moving for a vote of no confident against Harriet Jones? I'm not british, so I have very little idea of how Parliament governments work.