'Most' is the superlative form of 'many' or 'much'. 'Many' is defined as 'a large number of' and 'much' is defined as 'a large amount'. This naturally implies a statistically significant large majority, not a simple majority such as 51 to 49.
When there is a close division we don't say, 'most of the Senators' we say 'just more than half of the Senators'. This is because we haven't reached a point where we can adequately say that a 'a large number' (as compared to those voting against) 'has voted for'. For this reason we reserve 'most' for situations where more than a simple majority, e.g. often a supermajority, comprises one of the two or more things we are comparing.
Actually, grammatically speaking, superlative adjectives should _only_ be used when three or more things are being compared, however this is a commonly ignored rule.
There is one final point, when one of the two or more things you are comparing reaches a point where it is greater than two thirds of the total, we begin to say 'nearly all,' "Nearly all of the Senators voted to pass the finance reform bill today, voting 89-11."