I guess that you're then willing to stipulate that the far left is the reading public?
You said:
Stewart plays to a limited market, like Limbaugh, he's targeting the far left. He'll sacrifice sheer numbers to provide a high number of faithful who hear stuff catered directly towards them.
Item:
Also, his book, America (The Book): A Citizen’s Guide to Democracy Inaction, co-written with Daily Show executive producer Ben Karlin, topped The New York Times Bestseller List for 15 weeks.
Larson, Megan. "Comedy Inks Deal with Stewart's Busboy." MediaWeek, 15 Feb 2005. Available http: http://www.adweek.com/mw/news/cabletv/article_display.jsp?vnu_content_id=1000800423.
15 weeks at the top of the NYT bestseller list is nothing to sneeze at, especially for a work of satirical non-fiction. (And no, that's not a contradiction; see the Library of Congress Cataloguing In Process information.) I can't get there from here right now, but I can get the total number of weeks it's been on the list, not just at the top. This would indicate a sizable number of the reading public is reading this book.
So, if Stewart is targetting the far left, as you say, at the expense of a broader audience, and his book tops the bestseller list for almost 4 months, this means one of three things:
a) the far left makes up a disproportionately large portion of the reading public (unlikely: probably statistically in line with their representation in the general public); or
b) there are vastly more members of the far left than anyone thinks (even more unlikely: wouldn't we see them?); or
c) Jon Stewart appeals to a broader portion of the public than the right feels comfortable admitting, and so is trying to underplay his appeal by painting it radical (certainly in line with other attacks on adversaries).
So which is it?