  Samit Dasgupta has proved a formula factoring a certain restriction of a
3-variable Rankin-Selberg $p$-adic $L$-function as a product of a 2-variable
$p$-adic $L$-function related to the adjoint representation of a Hida family
and a Kubota-Leopoldt $p$-adic $L$-function. We prove a result involving Selmer
groups that along with Dasgupta's result is consistent with the main
conjectures associated to the 4-dimensional representation (to which the
3-variable $p$-adic $L$-function is associated), the $3$-dimensional
representation (to which the $2$-variable $p$-adic $L$-function is associated)
and the $1$-dimensional representation (to which the Kubota-Leopoldt $p$-adic
$L$-function is associated). Under certain additional hypotheses, we indicate
how one can use work of Urban to deduce main conjectures for the
$3$-dimensional representation and the $4$-dimensional representation. One key
technical input to our methods is studying the behavior of Selmer groups under
specialization.
