German drug and chemical giant Bayer AG, which last week announced plans to acquire GMO company Monsanto for $66 billion, has hired big-name lobbyists to advocate for the proposed merger, including the firms of former House Majority Leader Dick Gephardt (D-Mo.) and veteran GOP lobbyist Charlie Black.

The proposed merger is the largest so far in 2016, and is expected to receive intense scrutiny from antitrust regulators. Monsanto, based in St. Louis, is the world’s largest supplier of genetically modified seeds.

Gephardt Government Affairs, GOP-led advisory shop Cloakroom Advisors, and lobby firms Cassidy & Associates and Prime Policy Group are working either directly or indirectly for Bayer to lobby on the federal review of the acquisition, according to lobbying disclosure filings. The firms have ties to top lawmakers from Missouri. The filings do not specify fees associated with the work.

The lobbyists include Black; Gardner Peckham, former adviser to former House Speaker Newt Gingrich; Jordan Bernstein, chief of staff to former Rep. Jo An Emerson (R-Mo.); and Gregg Hartley, former chief of staff for Sen. Roy Blunt (R-Mo.).

They did not immediately return requests seeking comment Wednesday.

Monsanto has not hired any new lobbyists since the deal was announced, according to public filings, but employs a prominent team of lobbyists, including former Sen. Blanche Lincoln (D-Ark.) and the lobby giant Akin Gump. The company spent $4.3 million on lobbying in 2015, according to the Center for Responsive Politics.