“Will you contend for Baal? Or will you yourselves save him?” It is supposed to be the god who protects the people, not the other way around. The word for “contend” is yerub (the imperfect jussive verb), “let him contend,” and that is where “Yeru-Baal” comes from, using the “b” that ends yerub as also the “b” that begins “Baal.” The Hebrew verb translated “contend” is reeb (#07378 רִיב), related to the noun, #07379, cp. Hos. 4:1). It has a wide range of meanings, including both legal and non-legal meanings. It can mean to conduct or decide a legal case or carry on a legal dispute with, or it can simply mean to struggle, strive, or quarrel with. Here in Judges 6:31 it has legal overtones. Joash is saying something akin to, “will you contend for Baal in a court, will you open a legal case and dispute for him?” He is not making the simple statement, “Will you fight for Baal?” Joash goes on to say that if Baal is really a god then he can defend himself if someone has destroyed his property and his honor.
“Let the one who will contend for him, be put to death by morning.” Understood the way it is translated in the REV, Joash is saying that if someone is going to defend Baal then he should be executed because if Baal is really a god he will defend himself and needs no human intervention. However, the same Hebrew sentence can be translated, “The one who contends against him will be put to death before morning,” meaning that if Baal is really a god, then he will defend himself quickly and the offending party, in this case Gideon, will die very quickly. So the two meanings of the sentence are: that if Baal is a god he needs no defense and anyone who suggests he does should be put to death; and if Baal is really a god he will defend himself and put the guilty party to death very quickly.
Joash is in a tough spot. He wants to support Baal and not overly offend the people of the city (after all, the altar of Baal was on his property), but he also wants to defend his son and keep him from being killed. The ambiguous way he spoke was a delicate way of supporting both positions. They both allow for Baal to be recognized as a god and they both have the end result of protecting Gideon. Also, if Baal is angry and wants to defend himself, that option is open to him.
“let him contend for himself.” This same idea, and the same verbal root, is in Psalm 74:22 when the psalmist asks God to plead His cause. Also, in Psalm 35:1, David asks God to contend against the people who contend against David.