A new poll on Louisiana's U.S. Senate race shows it's a toss-up between Democratic incumbent Mary Landrieu and Republican challenger Bill Cassidy.

Each had 47 percent of the vote in a potential run-off. That's according to Public Policy Polling. Pollster Tom Jensen gives a slight edge to Cassidy, because of Landrieu's poor approval ratings.

Most of the time when you see an incumbent with those kinds of approval numbers, they usually don't get re-elected.

"Senator Landrieu's approval rating is only 42 percent — 52 percent of voters disapprove of her," Jensen said. "Most of the time when you see an incumbent with those kinds of approval numbers, they usually don't get re-elected."

Landrieu could possibly avoid a run-off if she received 50 percent or more of the vote in the primary, Jensen said. But the survey projects the three-term senator will get 44 percent of the vote in a primary race featuring Cassidy and Republicans Rob Maness and Paul Hollis.

"Even with 17 percent of voters still undecided, it's unlikely that Mary Landrieu can get to 50 percent in November, because among those undecided voters, only 8 percent disapprove of her," Jensen said.

Public Policy Polling, which is based in North Carolina, polled 664 registered voters last week.