Just on the substance, it's interesting to note the actual content of Rubio's "defense" of Obama as highly competent, albeit anti-American (or at least, anti-America as it has always been). The bill of particulars appears to consist mainly of the following:
1) Enacting the stimulus bill, which economists almost unanimously agree reduced the severity of the Great Recession, and which reflected standard U.S. macro policy, in response to economic downturns, since the 1930s.
2) Enacting Obamacare, which implemented a Heritage Foundation plan that Mitt Romney previously enacted at the state level and made the centerpiece of his 2008 Republican presidential campaign.
3) Supporting modest limitations on gun ownership, in accordance with the views of perhaps 70 percent of the American public.
4) Deliberately weakening the U.S. abroad, pursuant to a supposed view of U.S. power as evil. This claim by Rubio is paranoid conspiratorialism - the sort of thing that led William F. Buckley to expel John Birchers from the mainstream conservative movement. But now it is the credo of one whose claim to the "moderate" or "mainstream" slot among Republican presidential candidates is endangered only by the rising doubts that he can pass the Turing test.