Importance: Clinicians treating borderline personality disorder (BPD) are often faced with the difficult challenge of assessing when, and for whom, risk for suicide is greatest. Addressing this dilemma requires longitudinal, prospective data from high-risk samples with an elevated base rate of suicide attempts.Objective: To test whether dispositional characteristics modulate the pathway from interpersonal dysfunction to suicide in BPD. Design: This longitudinal, observational study was conducted between 1990 and 2020. Data were analyzed between April and July 2020. Participants were assessed annually for up to 30 years (mean number of follow-ups = 7.82). Setting: Participants were recruited from inpatient, outpatient, and community referral sources.Participants: 458 individuals (Mean age = 28.59, 77% female) diagnosed with BPD.Main Outcomes and Measures: Presence or absence of a suicide attempt within one year of each follow-up assessment. Multilevel structural equation modeling was used to 1) examine longitudinal, within-person transitions from interpersonal dysfunction to suicidal ideation to suicide attempts (i.e., the [I]nterpersonal-[I]deation-[A]ttempt pathway); and 2) evaluate whether two maladaptive personality dimensions, negative affect and disinhibition, moderated these transitions.Results: At the within-person level, there was support for the I-I-A pathway: suicidal ideation accounted for the association between interpersonal dysfunction and suicide attempts. Personality further moderated each component of the I-I-A pathway: negative affect was associated with a stronger coupling between interpersonal dysfunction and ideation; and disinhibition was associated with a stronger coupling between ideation and attempts. Conclusions and Relevance: The escalation from interpersonal difficulties to a suicidal crisis in BPD involves two psychologically distinct process. An internalizing process links interpersonal dysfunction to suicidal ideation and is facilitated by trait negative. An additional externalizing process links suicidal ideation to suicide attempts, and is facilitated by trait disinhibition. Assessment of these intra- and interindividual risk factors may inform clinical decisions about when, and for whom, crisis intervention is necessary.