Constitution Art.I Sec.5:"Each House may determine the rules of it's proceedings"

Before 1917 there was no provision,in the rules of the US Senate,for cloture.Therefore,a Senator or a small group of Senators could filibuster,thus preventing a vote,for as long as they wished.

The US Senate,in 1917,passed a cloture rule that required a 2/3rds majority of those "present & voting" to invoke cloture.

That cloture rule was in effect when the landmark 1964 Civil Rights Act passed the US Senate.

Every member of the US Senate was "present & voting" for the cloture vote on the 1964 Civil Rights Act.

Therefore,it would take 67 votes to invoked cloture,on the civil rights bill.The vote was 71 ayes & 29 nays.

44 Democrats & 27 Republicans voted aye.
23 Democrats & 6 Republicans voted nay.

Famously,Senator Engle of California,who had suffered a stroke,pointed to his eye as a "sign of his aye vote."

In 1975 the cloture rule was changed from 2/3rds of those "present & voting" to 3/5ths of those "duly & sworn".

Thus,regardless of those "present & voting",to invoke cloture, the rules require 60 votes.

Which is the cloture rule the US Senate operates under today.

But,because the Constitution says the US Senate " may determain the rules of it's proceedings" there is a proceeding,called Reconciliation,which allows the Senate,to bypass cloture & under a very strict set of rules,pass legislation on a majority vote.

Bernie backs down, again
"Given the very delicate situation at this time and the challenge facing Speaker Pelosi as she rounds up votes, Bernie and other senators have concluded that offering a public option amendment now could undermine the entire process," said Sanders spokesman Will Wiquist.