Not since the Old Gods at least. I'm not exactly sure before that.I've not played much as a republic, but my (relatively) recent Byzantine game saw Genoa dominating the southern part of the boot of Italy. Even when my Empire became strong, each and every war against Genoa was painful, as they'd instantly hire a bunch of huge mercenary companies. It was especially bad since usually those wars were to regain a single city in my vassals lands, so I'd deplete my levies and coffers in order for my vassal to gain a tiny bit of tax income, but dammit it's the principle that matters.
I must admit though, I do feel Republics are a bit overpowered at the moment. I've reached the stage where if I am anywhere in the Med I will make an effort to exterminate the republics as early as possible to avoid them dominating.
EDIT: also I swear there used to be a land bridge to Venice. Am I wrong?