However, when respondents were read information characterizing Fleming's voting record as more conservative than Cassidy's, Fleming leads Cassidy by nearly 20 percent in a head-to-head matchup, with 51 percent support to Cassidy's 31 percent support.
The poll, conducted by GOP pollster Glen Bolger of Public Opinion Strategies, also indicates that Fleming narrows Landrieu's lead to 5 percentage points in a runoff, the likeliest outcome of a race in which Fleming, Cassidy and Landrieu all run. Louisiana's "jungle primary" system means every candidate, regardless of party, is on the ballot; if two Republicans run, they're likely to split the vote and force a runoff against Landrieu.
And the poll offers evidence of just such an outcome. After opposition research points are read against Cassidy, Fleming and Landrieu, on a three-way ballot, Landrieu drops to 38 percent support, to Fleming's 32 percent and Cassidy's 20 percent support — an indication Fleming would move on in a runoff.
"Given Mary Landrieu's support of President Obama and his administration's failed policies, she is vulnerable. This is no surprise and echoes the sentiments that I hear as I travel Louisiana," Cassidy said.
Some Republicans have expressed concern that a runoff could spell bad news for the GOP candidate, as the party would be sending a battle-weary candidate to face Landrieu head-on.
But Brabender, who has worked with Fleming before and most recently served as former GOP presidential contender Rick Santorum's senior campaign strategist, said that "the polling data was very encouraging to" Fleming, and has increased his interest in potentially launching a campaign.
Brabender and Bolger are both on board with Fleming as he explores a possible bid.
Cassidy is known in the state to be preparing for a run, but some Republicans believe his decision to wait to make his bid official has increased the likelihood that another Republican contender will jump in the race.
It's unclear why Cassidy has waited, but he remains the least conservative of the three former and current members of Louisiana’s congressional delegation who are considering a run, measured by the Club for Growth's scorecard and National Journal's vote rankings.
Former Rep. Jeff Landry is also considering jumping into the race.
And Fleming's internal poll indicates Cassidy's less conservative record could be an impediment to him in a three-way race.
None of the top conservative groups have yet publicly weighed in on the race, but Brabender indicated Fleming could pick up support from some groups on the right who may be looking for the most conservative candidate in the race.
"There are many who feel that Louisiana has turned into a relatively conservative state, and getting conservatives and Tea Party supporters to rally behind a single candidate will be very very critical to Republicans picking up the seat. There's probably pretty strong agreement that Congressman Fleming's got conservative credentials," he said.
And he noted that, because Landrieu is one of the top GOP targets in the nation — President Obama is deeply unpopular in the reddening state — "there's going to be a lot of people interested financially."
Those outside funds would help boost Fleming, who currently has less than $500,000 cash on hand, compared to Cassidy's more than $2 million.
The survey was conducted among 500 likely voters from March 3-5 and has a margin of error of plus or minus 4.38 percentage points.