The reference of the title remains ambiguous at the closing credits. Does it refer to the posse of lawmen helmed by Douglas or the posse of outlaws being led by Dern?
I didn't find the title ambiguous at all. It refers to the posse that Douglas formed and that Dern took over. Early on (I think when we first see Douglas, but it's been years since I looked at this film, which by the way is one of my favorites) there's a picture of him and his posse which he or someone describes as "the most famous posse" ever. The marshall that Douglas plays is known for this efficient posse he formed and leads because they're good at tracking down and apprehending bad guys, especially those who rob the railroad that is backing Douglas's bid to become governor of Texas. Douglas is apparently considered a shoo-in for the office, which worries his posse about what's to become of them. Douglas says don't worry, the railroad will give you jobs as guards--at a cut in pay of course. To which the Indian lawman reminds him that the railroad doesn't hire Indians.
It's plain Douglas used the posse as a stepping stone for his political ambition but has no real loyalty to them. Outlaw Dern, however, admires their ability and efficiency, especially when they easily trick his less-savvy outlaw band into giving away their position by firing at too great a range and methodically move in and kill the outlaws one by one, with Dern the only one to escape.
Later when Dern captures Douglas, he prods the posse into collecting the money to replace his loot that burned in a fire. They rough up the townspeople but only come up with part of the money, at which point Dern says, "Close enough. Let's ride." At that point, most of the posse realize they've crossed the line from lawmen to outlaw and that the pay is better. And Dern does hire Indians, leaving Douglas's character trying in vain to track down and capture this efficient, deadly band of gunmen that he had made into a posse and that Dern remade into outlaws.
I read somewhere that Douglas was first cast to play the outlaw and Dern the ambitious marshall, but Douglas talked the director into reversing roles. Dern always made a really good badman.
His sworn posse is interested only in dollars and their own personal gains. In fact, during one of the film's most morbid moments, Bo Hopkins along with a couple of others in Douglas' posse elect to bed down a couple of teenage tarts in favor of listening to their boss' campaign speech. This brutal take on how selfish and corrupt human nature can be, fits right in with other 1970's revisionist westerns.
Now this I didn't find morbid or reprehensible at all! What red-blooded young man wouldn't prefer a roll in the hay to listening to anyone's political speech, especially knowing his boss is moving on up without him, based largely on the posse's reputation? Selfish? Corrupt? Revisionist? Naww! Since the beginning of time, boys and girls have slipped away to play slap and tickle. That's just human nature. For example, Audie Murphy, the most decorated hero of WWII, was just 20 years old and still recovering from wounds that ended his military career when he came back to his native Texas and was given a parade through downtown Fort Worth. But Murphy didn't show up for the big dinner given in his honor that night, with state and national politicians and generals and their staffs and wives all gathered to greet and meet him. Instead, Murphy was in his hotel room in bed with the elevator girl.
I think you were a little hard on
The Professionals, especially on Marvin and Lancaster. Marvin always put certain little nuances into each of his characters that make them different from each other, and as I've said before many times in this forum, he's the only actor who really looks like the gun he's carrying on screen is a real weapon. Other actors only look like they're carrying props.
Lancaster was always one of my favorites--just his presence and that toothy smile and the fact he was still so physical even at that age.
Yet having said that, if it came down to a showing of
The Professionals or
Posse, I'd go with
Posse.