The main cable anchorages of the Mackinac bridge are a long way from either shore. They are huge concrete blocks setting on the bottom. The saboteurs would have to cross several approach spans to get to the anchorage. The anchorages on the Golden Gate bridge are at the waters edge. It would be a major job to place enough explosive to part the main suspension cables of the Mackinac bridge.
Rather than bring it down, it would be a lot easier to plant several IEDs along the parade route and set them off simultaneously. I'm sure the terrorists have IEDs down pat by now, including their camoflage or even by using car bombs in the opposite lanes.
The casualties would be terrible in such a huge crowd, and the press would all be there to witness it, not to mention the Governor and numerous politicians. The carnage could be horrific, not to mention the added horror of those blown over the edge for the long, long trip down into the cold Straights,... you know, families and strollers and all that.