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Lifting homotopy T-algebra maps to strict maps

The settings for homotopical algebra---categories such as simplicial groups, simplicial rings, $A_\infty$ spaces, $E_\infty$ ring spectra, etc.---are often equivalent to categories of algebras over some monad or triple $T$. In such cases, $T$ is acting on a nice simplicial model category in such a way that $T$ descends to a monad on the homotopy category and defines a category of homotopy $T$-algebras. In this setting there is a forgetful functor from the homotopy category of $T$-algebras to the category of homotopy $T$-algebras. Under suitable hypotheses we provide an obstruction theory, in the form of a Bousfield-Kan spectral sequence, for lifting a homotopy $T$-algebra map to a strict map of $T$-algebras. Once we have a map of $T$-algebras to serve as a basepoint, the spectral sequence computes the homotopy groups of the space of $T$-algebra maps and the edge homomorphism on $π_0$ is the aforementioned forgetful functor. We discuss a variety of settings in which the required hypotheses are satisfied, including monads arising from algebraic theories and operads. We also give sufficient conditions for the $E_2$-term to be calculable in terms of Quillen cohomology groups. We provide worked examples in $G$-spaces, $G$-spectra, rational $E_\infty$ algebras, and $A_\infty$ algebras. Explicit calculations, connected to rational unstable homotopy theory, show that the forgetful functor from the homotopy category of $E_\infty$ ring spectra to the category of $H_\infty$ ring spectra is generally neither full nor faithful. We also apply a result of the second named author and Nick Kuhn to compute the homotopy type of the space $E_\infty(Σ^\infty_+ \mathrm{Coker}\, J, L_{K(2)} R)$.