This excerpt came from The Confidence-Man (1857) from Herman Melville:
Oh, the cripple. Poor fellow. I know him well. They found me. I have said all I could for him. I think I abated their distrust. Would I could have been of more substantial service. And apropos, sir," he added, "now that it strikes me, allow me to ask, whether the circumstance of one man, however humble, referring for a character to another man, however afflicted, does not argue more or less of moral worth in the latter?
It seems to mean "I would or I could", but I am not sure, is this an idiomatic phrase? I doubt it. What do you make of it?