The following is the section of the Foreign Service Act of 1980 (22 USC 3944) dealing with ambassadorial appointments: 

SEC. 304 - APPOINTMENT OF CHIEFS OF MISSION

(a)(1) An individual appointed or assigned to be a chief of mission should possess clearly demonstrated competence to perform the duties of a chief of mission, including, to the maximum extent practicable, a useful knowledge of the principal language or dialect of the country in which the individual is to serve, and knowledge and understanding of the history, the culture, the economic and political institutions, and the interests of that country and its people.

(2) Given the qualifications specified in paragraph (1), positions as chief of mission should normally be accorded to career members of the Service, though circumstance will warrant appointments from time to time of qualified individuals who are not career members of the Service.

(3) Contributions to political campaigns should not be a factor in the appointment of an individual as a chief of mission.

(4) The President shall provide the Committee on Foreign Relations of the Senate, with each nomination for an appointment as a chief of mission, a report on the demonstrated competence of the nominee to perform the duties of the position in which he or she is to serve.

(b)(1) In order to assist the President in selecting qualified candidates for appointment or assignment as chiefs of mission, the Secretary of State shall from time to time furnish the President with the names of career members of the Service who are qualified to serve as chiefs of mission, together with pertinent information about such members.

(2) Each individual nominated by the president to be a chief of mission, ambassador at large, or minister shall, at the time of nomination, file with the Committee on Foreign Relations of the Senate and the Speaker of the House of Representatives a report of contributions made by such individual and by members of his or her immediate family during the period beginning on the first day of the fourth calendar year preceding the calendar year of the nomination and ending on the date of the nomination. The report shall be verified by oath of the nominee, taken before any individual authorized to administer oaths. The chairman of the Committee on Foreign Relations of the Senate shall have each such report printed in the Congressional Record. As used in this paragraph, the term "contribution" has the same meaning given such term by section 301(8) of the Federal Election Campaign Act of 1971 (2 U.S.C. 431(8)), and the term "immediate family" means the spouse of the nominee, and any child, parent, grandparent, brother, or sister of the nominee and the spouses of any of them.