All of this is fantasy and foolishness. These senators might succeed in forcing NASA to spend money on the heavy-lift rocket that Congress has mandated, but there is no way the space agency will ever get enough funding or time to finish it. Even if the lower estimates are right, the cost is exorbitant, many times that of what the private companies have spent for their rockets and ships. And if construction does begin in earnest, it cannot be finished before the arrival of a new President in 2016 (at the latest), who, like all new Presidents, will have his own plans and will not want to build something started by the previous administration.
Much better to end this farce and save the money, especially considering the debt of federal government.