I don't think you can look at this in isolation; you have to remember the context of the wider war. Roosevelt and Marshall certainly had a lot more on their plate than what was viewed in the US as a local setback in the Philippines. There was a wider Japanese offensive (that still would have taken place no matter how successful MacArthur might have been on Luzon) in the Pacific that had to be arrested. The Coral Sea was fought in May, and Midway in June. The Japanese seized bases in the Aleutians during the battle of Midway and that impinged on the JCS directive to defend the Strategic Triangle.

In the ETO, the British were holding on by their finger tips in North Africa, and something had to be done to aid them; Operation Torch turned up on the JCS radar screens in August, IIRC. And there was the punishing U-boat campaign off the East Coast of the US that was seriously bleeding the Allied shipping pool. That doesn't even include the situation in the Soviet Union, which both Churchill and Roosevelt were worried about keeping in the war. Given these issues, and others, it was not surprising that Roosevelt agreed with his advisers that the Philippines weren't worth the effort to reinforce them. That was especially true since it was plain to see that any reinforcements the US could reasonably expect to send in the near future would only sustain continued resistence for a limited period, certainly not long enough to save the Philippines from eventual surrender.

In June/July, 1942, the long term US plans envisioned for the Pacific included a slogging match in the Solomons for the rest of 1942, and a Central Pacific drive that wouldn't start until the end of 1943. Under the best of circumstances, neither of these initiatives would get anywhere near the Philippines for at least another two years and even that estimate depended on things breaking the Allied way both in the PTO, and in the ETO. Even with substantial reinforcements, the most optimistic planners could not see Mac holding out on Luzon for anything like two years.

And the world-wide shipping shyortage weighed heavily on Roosevelt's mind; every one of the Allies was screaming for more ships, and they were painfully slow in arriving from the building yards. Roosevelt couldn't justify wasting twenty or thirty merchantmen on a gesture when it would not contribute to the ultimate victory.

As for the domestic political aspect you mention, it simply wasn't there. Very few in the US thought of the Philippines as exactly being "US soil" especially since Tydings-McDuffie; in fact, a substantial portion of the US public breathed a sigh of relief when that act was passed precisely because they viewed the Philippines as a defense liability and now that burden would shortly disappear. Also Roosevelt and the JCS, exercised quite a bit of influence over the US media; they could spin deveopments in the Philippines pretty much however they wanted, especially if Mac appeared favorably in the account. So, no, denying reinforcements to the Philippines, even if Mac achieved some sort of local success, wasn't "political suicide" for Roosevelt. In fact, to do so would have won him the support of the Army and Navy planners, as well as that of our British allies; something that may not have been crucial to Roosevelt, but was certainly useful political capital

The fact was, that no one in the entire US defense establishment believed retention of the Philippines was crucial to Allied victory, except perhaps MacArthur. The whole rational of the effort to reinforce the Philippines in the latter half of 1941, was not to retain them in case of Japanese attack, but to deter that attack from happening in the first place. When it became obvious that deterrence wasn't in the cards, the attitude in Washington was, well, that didn't work, so let's not throw good resources after bad. Of course, everybody felt really bad about the troops who had just been condemned to death or long-term imprisonment as POW's, but most military men who were in the know as to what was going on, advised that no further resources be wasted on the Pacific; Eisenhower, during his stint on the planning staff, even suggested that Australia was not critical to Allied victory, but was overruled by Marshall and Roosevelt for political reasons revolving around relations with the Brits.

There simply was not enough reason in the Allied camp, for anyone to argue strongly for the reinforcement of a hopeless cause already far behind Japanese lines, when that reinforcement would cost precious resources which would yield far better results in a multitude of other places.