The Senate began to see Augustus as one of their own after he had won their trust, and later only after hesitations because he far outranked them in power and prestige. From this, we can see that the Senate considered Augustus a powerful ally of sorts, and Augustus needed the Senate to uphold, on the surface, the fundamental premise of Roman political thinking, since anyone of importance in the political world was a part of the Roman Senate.
Under Augustus, many of the old senatorial families died, and the rest relied on him for survival, both financially and politically. Augustus also replaced much of the Senate with his chief supporters, or men in parts of Italy originally not represented in Rome. Those men tended to give their loyalty to Augustus, who made their full participation in politics possible. The Senate was ultimately transformed into a body that represented all parts of the empire.
The census was one way to reduce the size of the Senate, though it wasn't the main way. Obviously unworthy men were forced to leave (men whose backgrounds deviated too markedly from the norm). Augustus attempted a policy of encouragement, and then tried to make the senatorial class kick out those members. When neither worked, he had to force them out himself. The main purpose of these cuts were to wipe out the opposition from the Senatorial families with distinguished records.
Augustus measured "worthiness" by each man's net worth and corresponding income; the amount of wealth was 400,000 sesterces before Augustus - now it was 1 million sesterces. From the reform on-wards, only men already entitled to wear the tunic with the purple stripe could present himself as a candidate, wiping out the tradition of all Romans having a chance. The lower participation in elections mainly...