P.T. Barnum supposedly said, "You can fool some of the people all of the time, and all of the people some of the time, but you cannot fool all of the people all of the time." Who, however, said, "You can fool too many of the people too much of the time"?

James Thurber

Actually Abraham Lincoln was the original sayer of the above first quote:

http://www.quotationspage.com/quote/27074.html

Sorry, barker, but it is not at all clear who said it, which is why I used the word "supposedly." As qualified historians have pointed out, no documentation has ever supported the attribution to Lincoln, and the Barnum attribution is also disputed. My guess is that a journalist came up with it.

But yay, MrsAce! My hometown boy, humorist James Thurber, did publish the second remark, in "The New Yorker" on April 29, 1939.

https://en.wikipedia.org/wiki/James_Thurber

I found a site contributing the quote to Lincoln, do you have a site that contributes it to Barnum?

Many. Here are two in which the lack of a clear attribution is noted:
http://members.tripod.com/~mr_sedivy/quotes17.html
worldofquotes.com/topic/Deceit/1 no longer exists

Spofford, incidentally (mentioned in the first site), would have been Ainsworth Spofford, 6th Librarian of Congress (1864-97), appointed by Lincoln himself. Under him, the LC's collection of Lincolniana grew precipitously.
https://en.wikipedia.org/wiki/Ainsworth_Rand_Spofford

And yet another one which shows a lacking in clear attribution.

http://en.wikiquote.org/wiki/Abraham_Lincoln#Misattributions

Apparently Lincoln was cited as saying it in a speech in 1858, but there are no records of this.