Halliday & Matthiessen (2014: 700-1):
The command ‘vote against ...’ is realised metaphorically by a hypotactic clause nexus; it is realised as if it was a report of what the speaker says. This is just like the metaphorical realisation of modality of the explicitly subjective orientation. Thus the reported command can be tagged: I urge you to vote against ... will you? In other words, just like modality, speech function can be represented as a substantive proposition in its own right; and this proposition is a figure of sensing or saying that projects the original [i] proposal or [ii] proposition.