
Let's hope this isn't the end...
The probe's been designed (and the design works been paid for), built (and the buildings been paid for), and is ready to go (the launch may or not have been paid for - not sure. I'd say it probably has been paid for).
Enter OMB, stage right. Due to the Senate mandating SLS without actually allocating extra money for it, the funds have to come from the existing NASA budget.
Which is where OMB comes in. In an effort to keep NASA from breaking its budget, they'd like to scrap/limit, reduce its various upcoming probes/observation missions, thus breaking many of NASA's prior commitments to ESA, thus killing reasonably cheap Mars exploration for a decade, thus firing/scattering the brains behind the programs. 10 years from now, that pool of talent will have to be rebuilt, another series of probes designed, built and launched.
And all so a horribly expensive rocket (the first of which will cost 100 times more than the commerical alternative) that's never going to fly can get first place at the pork barrel...