“I wasn’t born yesterday” means that the speaker is not so naive as to believe certain faulty arguments or proposals. “I wasn’t born Republican, Democrat, or yesterday” means that the speaker doesn’t believe in the political argument (although the speaker seemingly keeps an independent mindset).
 
The saying “I wasn’t born Republican, Democrat, or yesterday” has been cited in print since at least 1987 and has appeared on bumper stickers and T-shirts. The political parties are often flipped in order, as in: “I wasn’t born Democrat, Republican, or yesterday.”
   
   
CafePress
I wan’t born Republican, Democrat, or yesterday.
(bumper sticker)
 
Zazzle.com
I wasn’t born
DEMOCRAT
REPUBLICAN
Or
YESTERDAY
(T-shirt)
   
16 November 1987, Dallas (TX) Morning News, “Women to flood the ballot” by Carolyn Barta:
One of the buttons among the bumper stickers, T-shirts and other feminist regalia being sold outside the recent Feminization of Power rally here said: “I wasn’t born a Republican, Democrat, or Yesterday.”
 
4 September 2000, Newsday (Long island, NY), pg. A30:
A button I proudly wear says “I Wasnt Born Democrat, Republican, or yesterday.”
 
Tuesday, March 20, 2007
Bumper Stickers—Personality Warning Signals?
(...)
COMMENTS
A Jacksonian said…
“I wasn’t born Republican, Democrat, or yesterday”
6:59 PM, March 20, 2007  
   
Organizing for America
BEING THERE & OTHER JOYS
By GREG KLINE - Nov 5, 2008 10:41:38 AM ET
(...)
An elder I knew used to say “I wasn’t born Democrat, Republican or yesterday” and she also used to say “It’s never too late to do the right thing”.
   
Media Matters for America
Varney: “If you want to pile on the debt, vote Democrat”
September 03, 2010 4:55 pm ET
From the September 3 edition of Your World with Neil Cavuto:
COMMENTS
by The_Cat (September 03, 2010 4:59 pm ET) 14  
Mr. Varney, I wasn’t born Democrat, Republican, or yesterday.