Halliday & Matthiessen (2014: 472):
When the clause starts with and that or and this, with the that/this referring back to (some part of) the previous clause, the sense may be one of elaboration, particularly if the continuing clause is a ‘relational’ one:
||| [1] But we’ve got to find those || [=2] and that is the hard part. |||
The nearest hypotactic equivalent would be a non-restrictive relative clause, which is the hard part. Note that many such examples lie on the borderline between elaboration and extension. We have already noted examples of this indeterminacy between elaboration and extension from the other side, with non-defining relative clauses with who where the sense is ‘and + personal pronoun’…