The Roman Republic in History

[Cimon]

While this is entirely true, I'd say the seeds of that shift go back further than Marius - to Tiberius Gracchus. Specifically in creating, for the first time, a "class consciousness" not simply for plebians in general, but the very poorest in particular. It was this which Marius was able to draw on to recruit and his one-time ally Saturninus was able to whip up and nearly topple the Republic.

I agree with you here Quintus. The Gracchi are really the beginning of the end.

From the Gracchi onward, all the senators knew what type of legislation needed to be passed in order to stabilize the various situations that threatened the stability of the Republic, but the Republic was really viewed as a zero-sum game by those in the upper echelons of power. If one senator got the credit for something, then that meant no one else did. One of the major failings of the late Republic is the senators' inability to look past the desire to be primus inter pares to the fact that, without solving any of the Republic's problems, the first among equals was actually going to be an emperor, and the "equals" were not going to be equals at all.