U.S. Sen. Chuck Grassley, R-Iowa, said Thursday he was "dropped" from the conference committee that will work on reconciling the different tax cut bills that passed the House and Senate.

In an early morning tweet directed to President Donald Trump, Grassley noted that he was excluded from the group even though he's the senior member of the Finance Committee.

He's also a former chairman of the committee.

"@realDonaldTrump I'm the most Senior member of the Senate Finance Comm I was dropped as Conferee So I won't be in front line fighting for what u and I believe to cut taxes," Grassley wrote on Twitter.

Michael Zona, a Grassley spokesman, said that no reason was given for the exclusion, even though Senate Majority Leader Mitch McConnell, R-Kentucky, notified the Iowa Republican before the announcement was made.

Zona said they couldn't speculate on why Grassley wasn't included but added "Sen. Grassley was notifying the president that he wouldn’t be on the conference committee because he and the president have talked so much about getting tax reform done."

Grassley has been the subject of criticism this week after a comment he made defending a provision in the Senate tax bill that gave more favorable tax treatment to large estates.

In a Des Moines Register article, Grassley said eliminating the estate tax would recognize the efforts of "people that are investing as opposed to those that are just spending every darn penny they have, whether it’s on booze or women or movies."

The comment brought a storm of protest. Later, the senator issued a statement saying his remarks were taken out of context and he was trying to make the point that the tax code should not penalize frugality, saving and investment.

Sens. Orrin Hatch, R-Utah, Mike Enzi, R-Wyoming, Lisa Murkowski, R-Alaska, John Cornyn, R-Texas, John Thune, R-South Dakota, Rob Portman, R-Ohio, Tim Scott, R-South Carolina, and Pat Toomey, R-Pennsylvania, were picked for the conference committee.

Hatch, Enzi and Murkowski are chairs of the Finance, Budget and Energy and Natural Resources committees.

Enzi, Cornyn, Thune, Portman, Toomey and Scott are on the Finance Committee.