On the other hand, once the Shuttle is shut down, there will be more psychological pressure to get a new vehicle launch-ready (whether it's Orion or Dragon or whatever), simply because of the realization of "OMG the USA has no access to space". This urgency won't be fully felt by politicians as long as the Shuttle is still on the table.This is rather like how the Nixon (and Ford) administrations felt no urgency to get the Shuttle flight-ready until after they realized that the previous Apollo hardware had run out (Skylab and Apollo-Soyuz) and that there would be NOTHING manned until the Shuttle could be ready. Of course, this still means we probably won't see any manned launches till 2016 or so, but the politicians won't stop procrastinating until they actually SEE that "we got nothing".
Personally, I think that the number one reasonably-likely-to-happen thing that could convince the US government to pour lots of money into NASA (as in "a budget exceeding $30 billion in 2010 dollars") would be if China mounts a credible manned lunar program (especially if said program includes a long-term lunar outpost). If China claimed that they would put men on the Moon by 2020 and the USA believed them, then you can be sure that a NASA astronaut will be there first to offer the Taikonauts a broom to brush off the moon dust from their suits.
In sum, getting funds for NASA is all about making the majority of Congress (or the President himself) seriously believe that giving more money to NASA will gain more political points (and money/jobs/influence for their district) than spending the money on the military, roads, or the multitude of other multi-billion-dollar projects that are routinely approved. For comparison, the war in Iraq has officially eaten up more money than NASA's entire existence from the 1950s till the present, and unofficially has cost even more than that, which just goes to show you where political priorities lie.