A Query for Political Historians Among Us

In the history of the modern televised convention, has any major candidate lost significant polling ground in the week prior to the convention?

It’s shocking to me that Obama, headed into what should be a glorious public display of support and a great coming together of the Clinton-Obama horde, seems to have dropped 7-8 points in national polling in just the past ten days. I wonder if this has any historical precedent?