And whan she came bifore kynge Arthur, she told from whome she came, and how she was sent on message vnto hym for these causes.
Publicly reiterating and supporting the official party policy; conforming to the party's agreed policies
If a politician is on-message, they say something that follows the official policy of their party. a politician who is on-message says things that are in agreement with the ideas of his or her political party, especially when it appears that he or she is not thinking enough about these ideas off-message