As I previously posted, it looks like the James Webb Space Telescope faces an uphill battle just to avoid the scrap heap. Today the American Astronomical Society released a statement on the proposed cancellation of JWST. To no one's surprise, the statement is strongly in favor of continuing JWST, making the argument that "Too many taxpayer dollars have already been spent to cancel the mission now; its benefits far outweigh the remaining costs", in addition to the more standard arguments about preserving jobs, maintaining America's leadership in space technologies, and the benefits to scientific research.
Essentially, I think the single most compelling argument for JWST is the "only way out is through" line of thinking. We've already spent over $3 billion on this thing, so we're pretty heavily invested. Additionally, the Hubble Space Telescope was 7 years late, cost three times as much as projected, and has been worth every penny, so there is precedent for this sort of situation working out well in the end.