Sen. Thad CochranWilliam (Thad) Thad CochranMcConnell tees up debt, government-funding vote National Flood Insurance Program is the next storm for hurricane survivors Trump exempts Citgo from Venezuela sanctions MORE (R-Miss.) rebounded in fundraising in the first quarter of the year, bringing in $1.7 million from January through March, while his primary challenger, state Sen. Chris McDaniel, slowed his pace.

 McDaniel raised $475,000, a decline from the $561,000 he raised in the fourth quarter, when Cochran brought in a disappointing $340,000. Cochran has $1.5 million cash on hand, while McDaniel hasn't yet released his cash on hand numbers.

Cochran is considered the likeliest incumbent senator to fall to a primary challenge this cycle because of his long tenure in the Senate and a rusty campaign operation.

His vulnerability has Democrats cautiously optimistic about their chances in Mississippi, as McDaniel has a history of controversial comments that Democrats believe could bite him in a general election, if he makes it through the primary. Democrats are running former Rep. Travis Childers in case of such an outcome.

But the new numbers from Cochran's campaign, in addition to a renewed offensive effort against McDaniel, indicate the Republican establishment has no plans to give the race up easily.

Cochran’s campaign launched a new ad on the same day hitting McDaniel for his voting record in local and national elections in Mississippi. The ad charges he voted in the Democratic gubernatorial primary in 2003 and didn't vote in the 2004 presidential election or the 2008 GOP presidential primary.

"If Chris McDaniel doesn't vote, why should we vote for him?" a narrator asks in the ad.

Meanwhile, the Chamber of Commerce waded into the primary on Tuesday as well, launching an ad that targets McDaniel’s career as an attorney. The ad charges he’s been “using the courts to seek an enormous payday,” and says he opposed court reforms “so he could score big money.”

“It’s not right, but it’s what trial lawyers like McDaniel do to get rich,” the narrator concludes.

In response, McDaniel accused Cochran of "desperate mudslinging," and called for the senator to agree to a debate.

"Sadly, Thad Cochran's recent campaign advertisement is the type of desperate mudslinging you'd expect from a career politician who has spent 41 years voting for higher taxes, higher spending, taxpayer funded abortion, and even to fund ObamaCare," he said. "My record as a conservative champion fighting for Mississippians' freedoms stands in stark contrast to Sen. Cochran's four-decade record of enabling government growth, and I hope Sen. Cochran will offer the people of Mississippi the debate they deserve." ​

The two will face off in the primary June 3.

—This piece was updated at 9:15 to reflect McDaniel's numbers.