  Our goal is to find an asymptotic behavior as $n\to\infty$ of the orthogonal
polynomials $P_{n}(z)$ defined by Jacobi recurrence coefficients $a_{n}$
(off-diagonal terms) and $ b_{n}$ (diagonal terms). We consider the case
$a_{n}\to\infty$, $b_{n}\to\infty$ in such a way that $\sum a_{n}^{-1}<\infty$
$($that is, the Carleman condition is violated$)$ and $\gamma_{n}:=2^{-1}b_{n}
(a_{n}a_{n-1})^{-1/2} \to \gamma $ as $n\to\infty$. In the case $|\gamma | \neq
1$ asymptotic formulas for $P_{n}(z)$ are known; they depend crucially on the
sign of $| \gamma |-1$. We study the critical case $| \gamma |=1$. The formulas
obtained are qualitatively different in the cases $|\gamma_{n}|
  \to 1-0$ and $|\gamma_{n}|
  \to 1+0$. Another goal of the paper is to advocate an approach to a study of
asymptotic behavior of $P_{n}(z)$ based on a close analogy of the Jacobi
difference equations and differential equations of Schr\"odinger type.
