The present article is the first in a series whose ultimate goal is to prove
the Kotschick-Morgan conjecture concerning the wall-crossing formula for the
Donaldson invariants of a four-manifold with b^+ = 1. The conjecture asserts
that the wall-crossing terms due to changes in the metric depend at most on the
homotopy type of the four-manifold and the degree of the invariant. Our
principal interest in this conjecture is due to the fact that its proof is
expected to resemble that of an important intermediate step towards a proof of
Witten's conjecture concerning the relation between Donaldson and
Seiberg-Witten invariants (hep-th/9411102, hep-th/9709193), using PU(2)
monopoles as described in (dg-ga/9709022, dg-ga/9712005). Moreover, it affords
us another venue in which to address some of the technical difficulties arising
in our work on Witten's conjecture. The additional difficulties in the case of
PU(2) monopoles are due to the more complicated gluing theory, the presence of
obstructions to deformation and gluing, and the need to consider links of
positive-dimensional families of `reducibles' even in the presence of `simple
type' assumptions. Witten's conjecture should then follow from a final step
analogous to Goettsche's computation of the wall-crossing terms, assuming that
the Kotschick-Morgan conjecture holds (alg-geom/9506018).