Buzzfeed clipped audio of the commercial and posted it online Monday.
The vast majority of black voters who cast ballots in the Nov. 4 all-party primary voted for Landrieu. Richmond, who is African American, represents a New Orleans-based district.
During the midterm campaign, Republican congressional leaders said that that they do not intend to impeach Obama.
At the top of the ad, Landrieu says she approves the message. The ad was paid for by the Democratic State Central Committee, one of the only pro-Landrieu groups to hit the airwaves in the runoff. Pro-Cassidy groups, meanwhile, are on pace to spend more than $5 million on ads for the challenger, about 100 times what the state Democrats are spending.
The only other group to run ads for Landrieu is the Human Society Legislative Fund, records showed Monday.
Buzzfeed later reported that state Democrats and Landrieu were also running a radio ad claiming Cassidy "has endorsed a documentary which claims slavery was better for black folks than welfare." A Cassidy spokesman told Buzzfeed he did not know what the ad was referencing.
Cassidy is widely regarded as the clear frontrunner in Saturday's election. Observers are anticipating an even more conservative electorate than the one that gave Republican candidates a majority of the vote on Nov. 4.
Senate Republicans gained eight seats on Election Day. A Cassidy win would give them a 54-46 advantage when the new Congress is sworn in early next year.
In the post-Nov. 4 period, Republicans have redoubled their efforts to link Landrieu to Obama, who is very unpopular in Louisiana. Freedom Partners Action Fund, a conservative super PAC backed by the political network of the Koch brothers, released a new ad Monday in which a woman says a vote for Landrieu is "a vote for President Obama."
A Cassidy spokesman did not immediately respond to a request for comment on Landrieu's ad.
Updated at 6:39 p.m.