  In the covariant description of confinement, one expects the ghost
correlations to be infrared enhanced. Assuming ghost dominance, the long-range
behavior of gluon and ghost correlations in Landau gauge QCD is determined by
one exponent kappa. The gluon propagator is infrared finite (vanishing) for
kappa =1/2 (kappa > 1/2) which is still under debate. Here, we study critical
exponent and coupling for the infrared conformal behavior from the asymptotic
form of the solutions to the Dyson-Schwinger equations in an ultraviolet finite
expansion scheme. The value for kappa is directly related to the ghost-gluon
vertex. Assuming that it is regular in the infrared, one obtains kappa = 0.595.
This value maximizes the critical coupling alpha_c(kappa), yielding alpha_c^max
= (4 Pi/Nc) 0.709 approx. 2.97 for Nc=3. For larger kappa the vertex acquires
an infrared singularity in the gluon momentum, smaller ones imply infrared
singular ghost legs. Variations in alpha_c remain within 5% from kappa = 0.5 to
0.7. Above this range, alpha_c decreases more rapidly with alpha_c -> 0 as
kappa -> 1 which sets the upper bound on kappa.
