This paper presents a semantic analysis of the Spanish modal verbs deber ‘must’ and tener que ‘have to’, based on their deontic uses in parliamentary debates. These verbs have previously been described in terms of weak vs. strong necessity or internal vs. external obligation, frequently without the support of empirical data. In contrast, we argue that the notion of (inter)subjectivity is crucial for a proper understanding of these verbs. Our quantitative analysis, in which we examined deber and tener que according to five variables related to the semantic notion of (inter)subjectivity (tense, polarity, grammatical person, diathesis, source of the necessity), shows statistically significant differences between the verbs. Deber is more frequent in the conditional tense, in negated utterances as well as in passive/impersonal clauses, in the third person and when the source of the necessity is not the speaker. Tener que usually occurs in the present tense and in affirmative and active clauses, is equally frequent in first and third person and usually appears when the source of the necessity is the speaker. Based on these results, we conclude that tener que is used to convey a subjective attitude whereas deber is used to indicate intersubjectivity.