"Majority" can take a singular verb, as in your sentence, if the word "majority" refers to a particular number of votes, or the group supporting the treaty is looked at as a whole.
Here is a usage note from the American Heritage Dictionary*:
"Whenmajority refers to a particular number of votes, it takes a singular verb: Her majority was five votes. His majority has been growing by 5 percent every year. When it refers to a group of persons or things that are in the majority, it may take either a singular or plural verb, depending on whether the group is considered as a whole or as a set of people considered individually. So we say The majority elects (not elect) the candidate it wants (not they want) , since the election is accomplished by the group as a whole; but The majority of the voters live (not lives) in the city, since living in the city is something that each voter does individually."
Rachel _______ The American Heritage Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company.2003
I would like to add little bit more to Rachel's reply. There is a set of nouns, called "collective nouns", which may take either a singular or plural verb depending on how the speaker perceives the situation. In US English, these nouns most often are perceived as single units, and thus take singular verbs. However, in British English the situation is reversed.
Some collective nouns:
family government team group staff data (technically plural, but as a common synonym for "information" it is uncountable)
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