To Bush critics, no evidence of "links" will be enough. We have an unresolvable dispute because no theory we present about Hussein-Al Qaeda can possibly be Proven Beyond A Shadow Of A Doubt (because, no theory whatsoever about the world can be PBASOAD.) Conversely, the "I doubt it" stance of the critics can never be completely refuted, because the theory that "Saddam he no conncetion even if it looks like he did; whatever evidence we find, there are alternative explanations for" is unfalsifiable. (Of course, this is because it is based on FAITH: namely, the faith the Bush is a "liar".)
There are two main things you will hear if you try to push this info to a lefty.
One primary response tactic will be "move the goalposts". For example: suppose that (best-case) this information pans out and after one or two years, it becomes universally accepted that Iraq and "Al Qaeda" had the "operational relationship" described here. The problem is, even if that happens, lefties will change the criteria for "links". For example they'll say "okay fine, we acknowledge that Saddam funded Al Qaeda, but that doesn't mean he (a) knew about 9/11 (b) was going to *continue* to fund Al Qaeda (c) so did Saudi Arabia (d) etc." They will forget that they ever said "no links" and pretend that the whole time the controversy was about "behind 9/11". Move. those. goalposts.
Maybe solid info comes out later that he *did* know about 9/11 coming. "Well, so did our CIA [or whoever, according to whatever conspiracy theory], and they didn't do anything to stop it, should we bomb them?" Move. those. goalposts.
Maybe solid info comes out that (say) the Salman Pak training facility was namely where the 9/11 hijackers practiced. "Yeah but big deal, they could have learned to hijack a plane with box-cutters *anywhere*." Move. those. goalposts.
Maybe solid info comes out that Saddam had a secret alliance with "Al Qaeda" ongoing, all the way through the March invasion. "Yeah but we forced him to stay in bed with Al Qaeda by threatening Saddam so much." Move. those. goalposts.
No amount of proof will suffice if your opponent is prepared to move the goalposts in response to everything you say. Leftists are. This is because Bush is evil and a liar and must be defeated. (The one and only Tenet of their faith.)
The second response tactic will be "but even if all this is true we didn't go to war BASED ON this information", why didn't Bush tell us this stuff from the beginning? The criticism will shift to the question of what Bush "used" to go to war and what he "didn't use" to go to war. Personally I find such arguments extremely irritating, because they seem to imply that there's some kind of Official List Of Reason We Are "Using" To Go To War.
One gets the idea that people who use such arguments would say that to call Al Capone a murderer and gangster is "unfair" because we only "used" his tax evasion as a reason to imprison him. In other words, it's a cheap legalistic game of "gotcha"; there's no real substance to the argument, but a lot of people sure seem to think that there is. We could get to the point that an iron-clad Saddam-"Al Qaeda" secret alliance is basically accepted by everyone, but lefties would still criticize by saying: "Bush should have told us that instead of 'Lying', then I would have supported him, but since he didn't, he's Evil and the war shouldn't have taken place."
In other words they'll still question the justification of the war on technical-rhetorical-legalistic grounds; even if it *looks* justified vis-a-vis straightforward national security by the discovery of the Saddam-Al Qaeda connection, lefties will say it wasn't justified because Bush said the wrong sentences in speeches; if only he'd said the magic words in speeches, lefties would have remained silent (or so they will claim)! I hope it's becoming clear just how silly I find such arguments, but I fully expect to hear them.
The point is that there is no convincing people who are True Believers, acting out of Faith.
You are so right. When a jury is confronted with an accused murderer confessing to beheading and chopping a man into small pieces, and exhonerates him for lack of evidence, what chance does this have?
But the "True Believers" are a slam dunk -- the Dems have another audience in mind: the legions of the uniformed. For these, justification in believing anything consists of "I heard in on TV," "I read it in a newspaper," or (lumping the totality of the leftist spin), "Everybody knows that." While many of these may be predisposed by ignorance or greed to believe Democrats, some are honest citizens who don't have the facilities (thanks to our public non-education system) to cope with seeds of doubt with which they are bombarded daily.
The last group -- which you might call the uniformed middle -- is the real target of the propagandists, and Clinton did the left a mighty service by staining the office of POTUS with personal corruption, making it easier for the masses to believe that successive presidents are nothing more than opportunistic liars.
The left understands Clinton's legacy well.