poll source                  dates administered democrat : tom harkin republican : christopher reed  lead margin
0                         survey usa      october 28 - october 29 , 2008                   61%                           35%           26
1                  rasmussen reports                   october 23 , 2008                   57%                           41%           16
2                  rasmussen reports                 september 25 , 2008                   54%                           40%           14
3                         survey usa  september 17 - september 18 , 2008                   60%                           37%           23
4  des moines register / selzer & co   september 8 - september 10 , 2008                   53%                           34%           19
5                  rasmussen reports                     august 7 , 2008                   60%                           36%           24
6                  rasmussen reports                      july 10 , 2008                   55%                           37%           18
7                  rasmussen reports                      june 10 , 2008                   53%                           37%           16
8                      research 2000          april 21 - april 23 , 2008                   59%                           20%           39

Table as JSON:
{"poll source":{"0":"survey usa","1":"rasmussen reports","2":"rasmussen reports","3":"survey usa","4":"des moines register \/ selzer & co","5":"rasmussen reports","6":"rasmussen reports","7":"rasmussen reports","8":"research 2000"},"dates administered":{"0":"october 28 - october 29 , 2008","1":"october 23 , 2008","2":"september 25 , 2008","3":"september 17 - september 18 , 2008","4":"september 8 - september 10 , 2008","5":"august 7 , 2008","6":"july 10 , 2008","7":"june 10 , 2008","8":"april 21 - april 23 , 2008"},"democrat : tom harkin":{"0":"61%","1":"57%","2":"54%","3":"60%","4":"53%","5":"60%","6":"55%","7":"53%","8":"59%"},"republican : christopher reed":{"0":"35%","1":"41%","2":"40%","3":"37%","4":"34%","5":"36%","6":"37%","7":"37%","8":"20%"},"lead margin":{"0":26,"1":16,"2":14,"3":23,"4":19,"5":24,"6":18,"7":16,"8":39}}