The ‘uh-oh’ moment for Boston politicos in the disastrous Martha Coakley campaign of 2010 came on the Sunday morning before the special election. Even before most citizens could begin to rub the sleep out of their pre-Mass eyes, the calls started coming in to ward bosses and precinct captains about Dot Ave. There were scores of red-and-blue Scott Brown placards grinning at them from storefront windows from Lower Mills to Columbia Road. The Coakley team had been caught flat-footed and it was all the worse because there was not a single Coakley sign to be had inside the 128 belt. They didn’t think they’d need them.