|Only non-partisan polls used, only one poll from each polling firm:
Pre-primary (2): Lamont -12.0%
Post-primary (3): Lamont -7.2%
Pre-primary (1): Martin -11.0%
Post-primary (1): Martin -6.0%
Pre-primary (1): Lunsford -12.0%
Post-primary (1): Lunsford +5.0%
Pre-primary (3): Cardin +6.4%
Post-primary (5): Cardin +7.2%
Pre-primary (3): Franken -3.0%
Post-primary (4): Franken -3.3%
Pre-primary (2): Tester +3.5%
Post-Primary (1): Tester +7.0%
Two months pre-primary (1): Merkey -13%
Just pre-primary (1): Merkley -3%
Post-primary (1): Merkley -9%
Pre-primary (2): Webb -13.6%
Post-primary (3): Webb -11.4%
In six of the eight cases, the eventual Democratic nominee saw his numbers rise after winning the contested primary. In one of the other cases, the "drop" was only 0.3%. In the only other case, Jeff Merkley saw his numbers rise 10% during the hotly contested primary period, even if they dropped after the primary. Merkley went on to win the election anyway.
There simply is no evidence that contested primaries hurt Democratic Senate candidates. If anything, the evidence is that these primaries help Democratic Senate candidates. This makes perfect sense, since having tons of free media, forcing your campaign into high gear, and testing your message is almost guaranteed to improve your chances in the campaign.
As such, why so many Democratic Party leaders seek to avoid contested primaries is difficult to comprehend. Perhaps they are dealing with pouty, high-name ID candidates who will only enter the campaign if the primary field is cleared beforehand. Whatever the reason, it would be a positive step forward for our party and our democracy if more people started realizing that contested primaries are, much more often than not, helpful for Democratic general election chances.