The state margin of error for the polling results is plus or minus 4.5 percentage points, suggesting the race may be closer.
Landrieu, 58, was first elected to the U.S. Senate in 1996.
Her 1996 and 2002 election campaigns were close, but she won a comfortable victory in 2008.
She has long been a part of Louisiana's political establishment. Her father, Moon Landrieu, was mayor of New Orleans.
Mary Landrieu served in the state legislature and as state treasurer before her election to the Senate.
She was an unsuccessful candidate for governor of Louisiana in 2005.
Cassidy, 56, is a physician and former state legislator who has served as a member of the U.S. House of Representatives since 2009.
His campaign centers on opposition to Obamacare and gun control legislation, and support for protection of Louisiana's coastal waters.