[tcolvin]

Graham;

I would say the mechanism was 'cold water', which the RAF liberally poured over all reports of dive-bombing accuracy and long-range high-performance fighters.
The RAF was for some time in a position to lecture the USAAF from a position of hard-won experience - Battle of Heligoland Bight, Meuse bridges and all that.
They could and did say "told you so" when the B17s and B24s were slaughtered in daylight.
The RAF poured water, I would suspect, by giving credence to the 'wired shut' story of the dive brakes on the A-36, which resulted after a couple of French pilots entered the dive before deploying the brakes and tore the wings off their A-36s. The absence of an anti-foaming agent in the hydraulic oil led to examples of A-36 dive-brake failure that the RAF also would have played up. I could go on (and on), but almost any book by Peter C. Smith will give you chapter and verse on the RAF's absolute refusal to accept dive-bombers, and the lengths they went to rubbish them.
The RAF air marshals were opinionated and bullying, and notorious for spin and political correctness. They brooked no dive-bombers or real cooperation with the army.

Which brings us to Portal's bitterly held opposition to the long-range fighter. If Portal was looking for one, as you say, then he had a very curious way of expressing himself.

I would refer you to Appendix G of Terraine's 'The Right Of The Line', titled 'Sir Charles Portal and the long-range fighter question', which begins; 'My repeated assertion of the direct involvement of Sir Charles Portal, while CAS, in the question of long-range fighters for the RAF, and his personal opposition to such a weapon has been questioned. Yet it is strongly documented in the Official History".
Terraine gives quotes;
May 27, 1941 Portal told the PM; "The long range fighter, whether built specifically as such, or whether given increased range by fitting extra tanks, will be at a disadvantage compared with the short-range high performance fighter". (NB the Mustang was flying by then).
June 3, 1941, Portal replied to Churchill's urging to increase the fighter's range, by repeating "that long-range fighters could never hold their own against short-range fighters".
In 1942, Webster & Frankland say Portal stated as a fact that the production of an aircraft with the range of a bomber and the performance of an interceptor fighter was a 'technical impossibility'.
Portal had arguments with General Arnold about the unacceptable (to Arnold but not to Portal) spectacle of 1,461 fighters inactive in Britain while USAAF bombers were being shot out of the skies. ('Told you bombers couldn't operate in daylight' was what the RAF brass thought, and probably said).
Portal remained unmoved even when Arnold "caused some Spitfires to be specially equipped to fly the Atlantic".
Terraine concludes his Appendix; "The advent of the P-51B in December 1943 settled the matter; it is clear that it constituted a blind spot in Portal's war direction".
I would add this was certainly not the only one.

The right horse for the course of tactical and strategic accuracy was the dive-bomber. Pace Portal, the A-36 could do both successfully, and even, it would appear, in the absence of aerial supremacy. German electricity generation could have been destroyed in 1943.
For Portal, however, the only imperative was the heavy bomber force and the mission to burn out the hundred biggest German cities.

Tony