Civil War if Boromir survived?

Supposing, that Faramir wouldn't try to stick it out with Denethor in order to gain the respect he's always wanted from Dear Old Daddy (and I don't think Boromir would betray Aragorn after being saved by him), I can't see how a civil war would be possible. Feeling both sides closing in on his rule, being supported by neither of his sons against Aragorn's claim to the throne, I also wouldn't be suprised if Denethor would commit suicide in this scenario as well.