new younger cat aggressively attacking older cat

3 weeks ago I rescued a 7 mos. old female cat who is possibly pregnant. I already have a 2 1/2 yr. old, spayed female. The introduction was gradual, I kept the kitten (Ellington)  in a separate room for the first week, bringing her out to meet Gracie a few times a day--and always supervised. They exhibited normal territorial behavior: hissing, growling, skulking around each other, but mostly leaving each other alone, there was no physical contact. After 2 weeks, Ellington was completely settled into the house. She is playful and cuddly with me, but is now very aggressive towards Gracie, physically attacking and chasing her around the house. She has bitten and scratched Gracie a few times. Gracie is now terrified, and hides under the bed. She no longer plays in the house and prefers to be in the yard. Four days ago I purchased the plug-in feliway, I very sternly yell at Ellington when she looks like she's about to attack, I spray water when she does attack, but she is not backing down. Gracie is miserable.  Will Ellington's behavior mellow out after she is spayed? I would love to keep her, but I'm not willing to do so at Gracie's expense. How long should I let the two of them figure it out. What else can I do?

[opus88]

oh thats too bad and you did all the right things with introduction too.
Maybe it is a hormone thing with her being pregnant?
I understand what you are saying abt can't jeopardize Gracie well being and her home, after all it is her home first.
but I so hate to see a rescue kitty being returned too.....I had same situation a few years ago and finally had to find another home for the newbie

I hope you continue to try to find solutions..........the two being the same sex is compounding the aggression.
Sorry I have no solutions either, I hope others will.

good luck