Derick Dillard -- who's married to JBD's daughter and Josh's sister, Jill -- tells TMZ ... Josh's guilty verdict in his child porn trial puts his father-in-law in a tough spot politically, and not just because of the obvious association to his son ... but because of public past stances he's taken on sex crimes. In Derick's eyes, Jim Bob's track record won't help.

Take, for example, a policy JBD proposed on a now-defunct site of his when he ran for U.S. Senate in '02, which talked about what should happen to somebody who rapes a woman. As you can see, he called for executing the perpetrator and keeping the baby alive.

It's not apples to apples, comparing that to what Josh's conviction for possessing a ton of child pornography -- but in Derick's eyes, it makes Jim Bob look hypocritical ... because he doesn't think his father-in-law would take as hard a stance now, in light of what's happening to his own flesh and blood.

More relative to Josh's situation is a bill JBD helped sponsor in 2001 -- when he was in the state's House of Reps. -- calling for widening the categories of sex offenses to include "pandering or possessing visual or print medium depicting sexually explicit conduct involving a child." Derick believes Jim Bob will get grilled on the campaign trail for that one.

Big picture ... Derick says Jim Bob's come across as less than transparent or credible due to vague statements he made while Josh was under investigation.

Of course, Derick's hardly a JBD fan to begin with. As you know, he and Jill have distanced themselves from the famous fam since 2017 ... when Derick accused his father-in-law of screwing his kids out of money.

Still, DD admits Jim Bob could gut out a victory, due to his TV fame and name recognition/wealth -- all of which could help him get past the PR nightmare of Josh.