Thanks for the posts. This is a good education for me. I was previously under the impression that Mac was always kicking and screaming for reinforcements and writing angry and somewhat paranoid letters to his wife when he didn't receive them.
The description of kicking and screaming is a tricky one, but I think McClellan can be excused the odd paranoid letter given the situation. To take the example of McDowell's corps:
McDowell's corps (then 1st Corps, AoP) was an integral element in McClellan's Peninsular plan of operations. It was his amphibious corps, containing the only division trained in making ampib landings, and IIRC he'd managed to pull together enough transport to keep the whole corps afloat at once. At the time McClellan left Washington, he had been promised by Lincoln that no more divisions would be removed from his command (after one of his corps was cut from three divisions to two).
As he was transferring his HQ to the Peninsula, the commander of the Washington defences (mendaciously) claimed he didn't have quite enough troops, and so McClellan's army was cut from four corps to three. Of the two corps that hadn't yet transferred to the Peninsula, he lost 1st Corps - the larger of the two formations that hadn't yet moved - along with all their transports, which were discharged as unnecessary. McClellan got this information while he was actively in the field with his troops, and after at least a fortnight of arguing he managed to get the amphib division (Franklin's) returned to his command, but by then it was too late and the Yorktown line was too strong for a single amphibious division to be able to outflank it.
McClellan continued to argue that he needed his force returned to the size it had been when he'd actually made his plans, and Lincoln came to the Peninsula in May. He didn't come ashore himself but sent a factfinding expedition ashore to see if McCellan really needed the extra troops represented by the rest of McDowell's corps (which had now expanded back to three divisions with the addition of Shields' division.)
The result of the factfinding expedition, which included cabinet members, was unanimously that McDowell's corps "with Shields' division" (i.e. all of it) should be sent by sea to join McClellan as soon as possible. (This result was returned about 24 May.)
Lincoln decided that sending McDowell by land would be better, and ordered McClellan to operate against the rail lines north of Richmond so that McDowell could come and join him. McClellan did this, but McDowell never arrived - the movement being repeatedly suspended but not actually cancelled, so McClellan had no idea whether McDowell would actually arrive.
(McDowell's opinion on all of this was that he should be sent to join McClellan as soon as possible.)
At the time McClellan's position was a bit precarious. He was spread over the Chickahominy river near Richmond, with his supply base north of the river while Richmond is south of it, so to attack Richmond while protecting his supply base he needed a large force both sides of the Chickahominy. Without McDowell's corps McClellan was essentially stuck
either attacking Richmond
or keeping a secure line along the Tolopatamoy Creek to stop Jackson coming down to attack him, and after weeks of waiting he eventually took a gamble and began attacking Richmond - possibly based on the idea that since McDowell was delayed trying to stop Jackson then hopefully Jackson wouldn't turn up, or possibly based on the idea that since the ground was now dry McDowell would finally be marching down to join him like had been promised for a solid month.
Jackson promptly turned up, which leads to days 2-3 of the Seven Days battle sequence and McClellan being pushed away from Richmond by quite possibly the largest Confederate attacking column of the entire war; certainly the army that Lee commanded during the Seven Days is the largest Confederate army to ever take the field of battle at the same time.
McDowell never arrived with McClellan, even in July.
Had McClellan had McDowell in late June, as he'd been promised a solid month before, the Seven Days battles would have been very different - Jackson's outflanking attempt would have run straight into McDowell's corps dug in along Tolopatamoy Creek, the supply base would have been safe, and the likelihood based on McClellan's
historical actions (he'd begun his regular approach sequence) is that Richmond would have fallen in the summer of 1862.
Since McClellan knew that Lincoln had taken McDowell from him, and then not returned the corps despite a promise to do so, and since McClellan also knew that Lincoln's military advisors were telling him to send McDowell, one can see why McClellan would not have been happy with Lincoln about this.