Given the fact that there is clear and irrefutable evidence that Donald Trump expressed support for the Iraq War in the early stages of the conflict he is achieving nothing by continuing to say that the opposite is true. In fact, he is only making himself appear foolish and pathological in his dishonesty.
While a case can be made that he wasn't particularly emphatic about his support at the time, Trump clearly said "Yes, I guess so." when directly asked if he supported an Iraq Invasion. There is recorded evidence of him making that statement on Howard Stern's radio show in 2002, a full six months prior to the actual war. He can explain or parse that statement as one he has since evolved from, but he cannot and should not continue to deny that he very clearly spoke that statement, in context, before the war began.
I think that Trump should absolutely admit to initially backing the Iraq war. Much of his campaign has been based on uncovering lies and false claims by Hillary Clinton, but for his own campaign to have validity, he needs to be upfront, honest and able to admit the truths about his own past.
Yes, Trump should admit he initially backed the Iraq War because there is proof he did. He told Howard Stern that he supported it at first, and we have the audio to prove it. How can he continue to lie? There is no reason he should continue to lie about his stance.