It depends. If the position you are looking at requires a security clearance, whether it is an internship or job, it is best to start as early as possible. The application deadline for internships that require a security clearance is typically at least 11 months in advance. If you are applying for jobs for after graduation that will require a security clearance, we suggest beginning applications as a junior, as the security clearance process can take anywhere between 6 month to 1 year or more.
If you are looking at NGOS or think tanks the timeline is more flexible. Jobs are posted more sporadically and timelines are shorter. Applications may only be due a few months or even a few weeks in advance of the position start date.
See the "Resumes and Cover Letters" page for tips on crafting a compelling cover letter and further resources that provide guidance.