Sen. Marco RubioMarco RubioEthiopia at tipping point as Congress mulls human rights bill Report: Trump to reverse Obama’s Cuba policy Report: VA drug thefts not going away MORE (R-Fla.), meanwhile, is slightly better known but still carries net-negative favorability. Some 32 percent say they have an unfavorable opinion of the freshman senator, versus 29 percent who hold a favorable opinion. A plurality — 39 percent — hold no opinion of Rubio.

The only candidate to rank strongly in voters' minds was former Florida Gov. Jeb Bush, with sentiments likely heavily influenced by impressions of his brother, former President George W. Bush, and his father, former President George H.W. Bush. Some 45 percent of voters hold an unfavorable impression of Bush, versus 36 percent who see him favorably. Bush's 19 percent unknown was far less than the other potential candidates.

Still, it's not unusual for vice presidential candidates to be generally unknown before their nomination to a party's ticket, with opinions quickly crystallizing in the weeks after their selection. And all of the surveyed candidates likely poll better in their home states — Florida and Ohio, expected to be among the most pivotal in November's election.