Having only one third of its members stand for election at a time means that the senate is

It means that the senate is
- less likely to be responsive to public opinion 
-. more likely to have stability and permanence 
- likelier to have its members become experts in their fields
Unlike president, the election for the senate happens every 2 years and only 1/3 of its members are up for election. This means that it won't be possible for senate body to had drastic change in policies because the majority of the seats will always be filled by old-timers.

Lodg'd with me useless, though my soul more bent