Sen. Richard Burr (R-N.C.) has long been considered one of the most vulnerable incumbents in the country, save for the fact that he did not have a big-name Democratic opponent. This remains true after today's Public Policy Polling survey, which finds the first-term senator continuing to hold off his three challengers -- all of whom are unknown to more than two-thirds of the state's voters.

The question is: how long will they remain unknown, especially with the national Democratic Party all but endorsing former state Sen. Cal Cunningham, who only recently entered the race. Cunningham is unknown to 81% of the state, while 80% aren't sure of attorney Kenneth Lewis and 69% of Elaine Marshall, who's serving her fourth term as North Carolina's Secretary of State.

As for Burr, nearly 30% still don't know enough about him to form an opinion, while 35% approve of his job performance and 37% disapprove. Tested against a generic Democrat, Burr holds a statistically insignificant 42%-41% lead. Here's how he tests against the three Democrats in the race:

Burr 45 - Cunningham 36 - Und 20

Burr 43 - Lewis 37 - Und 21

Burr 42 - Marshall 37 - Und 21

The PPP survey was taken of 593 registered voters in North Carolina and conducted from Dec. 11-13 with a margin of error of +/-4.0%.

Civitas Institute released a poll on the race yesterday, finding Burr leading Marshall by 8 points.