Jamelle Bouie reiterates the reality:
According to the latest national survey from Public Policy Polling, Rubio’s favorability with Hispanics is 35/42, a deficit of seven percentage points. What’s more, Romney’s support among Hispanics is virtually unchanged, regardless of whether Rubio is on the ticket. With the Florida Senator as a running mate, Romney wins 32 percent of Hispanic voters to Obama’s 67 percent. Without Rubio, Obama’s margin grows to 68/30. In other words, at best, Rubio holds Obama to his (outstanding) 2008 performance among Latino voters.
Larison considers the hype surrounding both Rubio and Paul Ryan as VP nominees:
Having Ryan as VP loses eight points among somewhat conservative respondents, and Rubio loses five. Moderate voters are a bit more likely to support Obama against either pairing. … It is useful to remember just how relatively unpopular and nationally unknown these two politicians are when so many Republicans are certain that one or both of them would be a great addition to the presidential ticket.
Relatedly, Silver finds that veeps are unlikely to swing a state:
[I]t seems likely that the vice presidential nominee’s effect on his or her home state is normally quite modest — perhaps two or three percentage points on average, if a little more in some cases and a little less in others. To be sure, two or three percentage points in the right swing state is not trivial, but it is probably not enough to outweigh the other strengths and weaknesses that a vice presidential candidate could potentially impart onto the ticket.