The exciting potential of orbital rendezvous in future manned space flight had largely ceased to be a matter for dispute in NASA after the middle of 1961. Some planners still hesitated to endorse rendezvous techniques as the basis for a lunar landing mission in Apollo, but none denied its long-term importance. Theory and experiment alike suggested that guiding two spacecraft to a meeting in orbit ought to present no special problems, but until the technique could be demonstrated doubts remained. Should rendezvous prove to be as trouble  free in practice as it seemed to be in theory, then it might be worked into planning for the trip to the Moon and allow that journey to be mounted sooner and more cheaply. In late summer 1961, this prospect inspired Chamberlin to propose a program for Mark II that would beat Apollo to the Moon.
Chamberlin first proposed using rendezvous in Earth orbit to allow Mark II to circumnavigate the Moon and followed that up with an even more daring scheme based on rendezvous in lunar orbit to land men on the Moon. This succession reflected the trend of thinking in NASA as a whole. The last half of 1961 saw the technique of lunar orbit rendezvous gain growing support as a means to achieve early manned lunar landing. But Chamberlin was moving far more quickly than his colleagues. Perhaps the greatest defect in his plans was that they assumed the rendezvous technique itself to need no special work, that a few flights would suffice to prove the technique before going on to apply it to larger ends. This was an assumption not widely shared, and both plans were rejected for Mark II, although Chamberlin may well have blazed the trail for rendezvous in Apollo.
This still left the development and demonstration of rendezvous maneuvers as a proper goal for the Mark II project, and that became the basis for Chamberlin's revised plan. This fitted NASA's clearly growing inclination to see a place for rendezvous in its lunar mission. Pressure for a rendezvous development program of some kind was becoming intense. Thinking about lunar orbit rendezvous for Project Apollo could only make the matter seem more urgent. There might be some room for error in Earth orbit, where a failure need not mean the loss of the crew. But that margin did not exist in lunar orbit; sound and fully proved techniques would be crucial.
By late 1961, a rendezvous development program may well have become inevitable, and Mark II was not the only candidate in the field. Phase A of Project Apollo itself and Marshall's orbital operations development program were likely rivals. The Mark II project, however, had a clear edge: a spacecraft already designed and very nearly ready to go into production and a set of sharply defined and suitably limited objectives. When NASA decided late in 1961 that it needed a rendezvous development program, Mark II was there.