According to legend, seafaring rats may possess an almost supernatural psychic ability to predict the future or to interpret natural lore such that they would have foreknowledge of the doom of the ship.  They would leave, swim away, and take their chances in the open water rather than going down with the ship.

Absent scientific data to confirm the legend, the metaphor "like rats leaving a sinking ship" seems to have taken on a life of its own that may be used to describe certain committed leftists who have interpreted the proverbial tealeaves or seen the "handwriting on the wall" regarding the Affordable Health Care Act, a.k.a. Obamacare. 

To wit: Montana Senator Max Baucus, the chairman of the Senate Finance Committee and one of the architects of Obamacare, expressed his sense of hopelessness concerning the financial implementation of Obamacare in a harangue directed at Health and Human Services Secretary Kathleen Sibelius in which he characterized the (not so) Affordable Healthcare Act as a "huge train wreck."