The admission of new members to the United Nations is decided by the General Assembly.
Normally, if a state wants to join, it presents its application to the Security Council first.
If approved, the Security Council gives its recommendation to the General Assembly.
Article 18 of the UN Charter states says that the final decision rests here: “Decisions of the General Assembly on important questions shall be made by a two-thirds majority of the members present and voting. These questions shall include… the admission of new members to the United Nations.”
Until now, a veto has never been used in the Security Council because only states that were certain of winning approval would apply to join. But the US has threatened to veto the Palestinian request.
At the moment the Palestinians have ‘observer’ status. It is possible that the General Assembly might treat the Palestinians in the same way that it did with the Vatican: upgrade them to a ‘non-member state with observer status.’
A majority of the 193 UN-member states in the General Assembly is said to support that option.