West Virginia Attorney General Patrick Morrisey will announce his Senate campaign on Monday afternoon, strengthening the field of Republicans lining up to challenge incumbent Sen. Joe Manchin (D.).

Morrisey wrote on his Facebook page Sunday evening that he would be making a "major announcement" in Harpers Ferry at 5 p.m. on Monday. National Review reported Monday morning that the major announcement would be a Senate run.

The West Virginia attorney general spearheaded a high-profile fraud investigation into pharmaceutical company Mylan and will now be running to unseat Manchin, whose daughter, Heather Bresch, is CEO of the pharmaceutical company.

Morrisey's probe into Mylan, a company that is Manchin's second largest source of federal campaign funds, centered around the skyrocketing cost of its EpiPen.

Morrisey demanded that Mylan turn over all documents related to the EpiPen, calling its actions "outrageous." A two-pack EpiPen injector cost $100 in 2007 and rose to more than $600 by 2016.

Manchin defended his daughter after she testified in front of Congress regarding the price increase, stating that her "heart is pure as can be."

His wife, Gayle Manchin, also ended up in the spotlight after it was reported by USA Today that, as a member of West Virginia Board of Education, she pushed to require that schools purchase EpiPens.

Manchin and his political action committee have received nearly $400,000 from Mylan in campaign contribution since 2010.

Morrisey announced he was seriously considering a Senate run in early June. He successfully won his attorney general reelection campaign 52 percent to 42 percent in 2016.

The West Virginia fraud investigation Morrisey led sought to review Mylan's internal company files documenting the rebates it paid to the state Medicaid program. West Virginia officials were questioning how the pharmaceutical company classified the EpiPen.

An inspector general report released in late May revealed Mylan overcharged the government by $1.27 billion for failing to correctly classify its EpiPen.

Mylan classified EpiPen as a generic drug instead of a brand-name drug under the Medicaid rebate program.

Mylan settled with the Department of Justice in October of 2016, agreeing to pay $465 million back to the government for failing to correctly classify the EpiPen.

Morrisey slammed the settlement as an "irresponsible," "sweetheart deal" for the pharmaceutical company.

"It does not nearly pay for the damage done by Mylan and sends the wrong message of leniency to other pharmaceutical companies engaged in the Medicaid market," Morrisey said in a statement."

"The people are fed up with such arrangements, and, frankly, they should be," he said. "West Virginia deserves better."

Morrisey will be running against West Virginia Rep. Evan Jenkins (R.) and coal miner Bo Copley in the Republican primary.