Hi, A.. I agree with soooo many of these moms' responses! The "Lemme know when you're done" thing? LOOOOVE IT! Always worked miracles for me! Literally turned my back, rolled my eyes, went about my business and shot 'em down. Even now, the MOMENT I hear one of the pre-teens go, "Moooom, so and so just...." I cut 'em off with, "And you aaaare?...." and they INSTANTLY (deflated, ohhh so deflated) respond, "tattling."
And they walk awaayyyy!
IMO (and my experience; 3 kids - 12,10 and 6 now) ANY reaction - especially an emotional one!!!! - is 'ATTENTION'. Even NEGATIVE attention is ATTENTION. Some kids even pursue this MORE because it's a more passionate FORM of attention!
IGNORE IGNORE IGNORE!!! (without actually ignoring her, you know what I mean).
She's tiny and young, sure. But obviously smart enough to know (so she thinks) how to get her way - with force. These little ones have soooo much more energy than we do.
Nowadays "sending to their room" is pointless. But, asking them to leave the room you're in the moment they "start"? Well, this works.
You're the leader. Not only are you conveying a silent (and strong) message on "what is acceptable and unacceptable behavior", you are ALSO TEACHING HER WHAT YOUR STANDARD OF ACCEPTANCE FROM ANYONE IS! Not only will she see you "deal with her", she will be privy to your powerful phone conversations with Nicor, Com Ed, in-laws (hehe).... any conflict-involved or serious conversation. She will learn your tone, know when you mean business and understand very quickly that "I don't care WHO you are, I (you) will be approached a CERTAIN WAY and that way is WITH RESPECT!"
More importantly - she will see and hear you keep a calm tone but still manage to be firm and express yourself well. She'll see how you "get what you want" without raising your voice, demanding, becoming unruly.... She'll learn what works and what doesn't work. :)
I've learned one thing solidly, religiously and permanently: I could "tell" my kids what they ought to know 'til I'm blue in the face. But it's what I DON'T realize I'm teaching them (how they watch me) that becomes ingrained in them.
Look on the bright side - you've got a future lawyer or political activist on your hands there! No one says you have to "cure" this aggressive side of hers, she just has to learn that there's a time and place. I personally love outspoken kids!!! And, again, yes - 20 months is young - but her mind? SHARP AS A TACK! It's okay for her to learn diplomacy at such a young age. After all, she's the one dipping her foot in that very pool! Good luck!