Let p be a polynomial in one variable. It is shown that the universal
C*-algebra of the relation p(x)=0, \|x\| \le C is semiprojective, residually
finite-dimensional and has trivial extension group.
We solve the lifting problem in C^*-algebras for many sets of relations that
include the relations x_j^{N_j} = 0 on each variable. The remaining relations
must be of the form \| p(x_1,...,x_n) \| \leq C for C a positive constant and p
a noncommutative *-polynomial that is in some sense homogeneous. For example,
we prove liftability for the set of relations x^3=0, y^4=0, z^5=0,
xx^*+yy^*+zz^* \leq 1. Thus we find more noncommutative semialgebraic sets that
have the topology of noncommutative absolute retracts.
We solve a class of lifting problems involving approximate polynomial
relations (``softened polynomial relations''). Various associated C*-algebras
are therefore projective. The technical lemma we need is a new manifestation of
Akemann and Pedersen's discovery of the norm adjusting power of quasi-central
approximate units.