Party discipline is often viewed as weaker in the Senate than in the House. The key reason for this is the underlying principle of ‘unanimous consent’ in the Senate – this affords individual Senators considerable power in actions such as the filibuster and (anonymous) holds. The latter was supposedly stopped in 2011 but there is still evidence that anonymous holds are used, and it did not stop the practice of applying holds, just that they shouldn’t be anonymous.
These two stories, however, demonstrate the power that the Senate Majority Leader, currently Mitch McConnell, can have. He has declined to allow a bill on Iran to come to the vote in the face of a likely Democratic filibuster. This decision was made more controversial by the fact that he had planned to schedule a vote before the vote had been through the committee stage of the legislative process, again suggesting that he does have some power.
In the other example, Democrats are trying to apply pressure to McConnell to allow a confirmation vote of Loretta Lynch to Attorney General, which has currently taken 117 days. Therefore the control that the Majority Leader appears to exercise over the day to day floor actions of the Senate provides a good counter-balance to the traditional argument of weak discipline in the Senate.