“Raising over $500,000 in the first quarter puts my campaign in an extremely strong position. I am so grateful to the many people who have generously supported me and my efforts,” Cassidy said in a statement.

He went on to preview the lines of attacks likely to develop in the Senate race against Landrieu, who faces a tough reelection fight in a state where President Obama remains deeply unpopular.

"Obamacare is a bad deal for the American people and we need to repeal it and replace it with a responsible alternative. Energy is a proven job creator -- let’s get government out of the way and let our economy grow and prosper again. The substantial support my campaign received shows that these goals have struck a chord with the people of Louisiana," he said.

The $500,000 sum is a strong start for Cassidy, and it's more than twice as much as he raised in the first quarter of the previous cycle. At the end of last year, he posted more than $2 million cash on hand.

Cassidy is known to be preparing for a race, and he has signed up prominent Louisiana consultants Jason Herbert and Scott Hobbs to help.

But his reluctance to officially launch a bid has caused some potential Republican challengers, including former Rep. Jeff Landry and Rep. John FlemingJohn Calvin FlemingCoast Guard suspends search for missing Ohio plane Freedom Caucus member to bring up bill on impeaching IRS chief GOP seeks to make it 52 MORE, to look more closely at the race. Fleming's exploratory campaign released polling in March showing Fleming would be competitive against Cassidy in a primary election.

But Cassidy's strong first-quarter numbers could dissuade any weaker fundraisers from making a run.