The three-term Democrat, considered among the most vulnerable for re-election this fall, faces questions over her residency and her use of Senate funds for flights.
Following reports last week that Landrieu may not qualify for residency in her home state, a former Republican contender has filed a formal challenge to her candidacy in court.
Paul Hollis dropped out of the Republican race and endorsed Republican U.S. Rep. Bill Cassidy against Landrieu in June. On Friday, Hollis filed a lawsuit contending Landrieu lives full-time in Washington and therefore does not meet requirements to represent the state.
"Mary L. Landrieu is, in reality, a full-time permanent inhabitant of the District of Columbia," the lawsuit reads. "By all measurable and legal standards, her actual domicile is her $2.5 million residence on Capitol Hill, the only home she owns."
Landrieu, who is registered to vote and pays taxes in Louisiana, lists her parents' New Orleans address as her official residency.
The house is owned by a trust shared equally by Landrieu's parents and their nine children, including the senator.
"I have lived at my home on Prieur Street most of my life and I live there now, when not fulfilling my duties in Washington or serving constituents across the state," Landrieu said.
On Tuesday, the Louisiana Republican Party fired another complaint at Landrieu, this time alleging she used Senate funds for campaign flights.
LAGOP Executive Director Jason Dore sent a letter to the Senate's Committee on Ethics, asking it to investigate whether Landrieu violated a federal law prohibiting official taxpayer funds for campaigning.
"Considering that Sen. Landrieu admitted to breaking the law on two occasions, and there is strong evidence of other occurrences, the Senate Select Committee on Ethics should undertake a full review of the Sen. Landrieu's travel expenditures from her official account to ensure that Louisiana taxpayers have not been unwittingly and illegally subsidizing her re-election campaign by funding her campaign travel," Dore wrote.
Landrieu admitted two flights from last year were paid for with Senate money, and the campaign has reimbursed her official fund for one of the flights and corrected what the office said was a "bookkeeping error" for the other.
Her office has ordered a review of all her flights, the results of which are due out this month.
Landrieu holds a solid lead over Cassidy in the polling for Louisiana's open primary, which takes place November 4. Polls have her averaging about 40 percent, over three Republican challengers, but far short of the 50 percent to avoid a head-to-head runoff December 6.
In two-way polling, Cassidy and Landrieu are in a virtual tie.