"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."
The American Heritage Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company.2003