Republican Rep. Bill Cassidy's campaign for Senate in Louisiana has hit a rough patch.

Fresh off a disappointing fundraising quarter during which his opponent, Democratic Sen. Mary Landrieu, brought in $1.35 million to his $700,000, Cassidy has been passed over for a major conservative endorsement by the Senate Conservatives Fund.

The group announced Monday that it will instead support Rob Maness, a retired Air Force colonel, over Cassidy, who is considered the Republican frontrunner.

"[Maness] offers voters a compelling choice over Sen. Mary Landrieu because he's not a Washington insider," said SCF Executive Director Matt Hoskins.

The endorsement could give Maness, who raised about $58,000 between July and September, a much-needed fundraising and publicity boost, as groups such as the Senate Conservatives Fund have done for candidates like Sen. Ted Cruz, R-Texas.

As a dark-horse Republican candidate, Maness might also be helped by Louisiana's open primary system, in which voters may choose any candidate, regardless of party, and the top two advance to the general election.