"They completely blindsided us. It's absolutely ridiculous," Moulton told CNN's Alisyn Camerota Tuesday on "New Day." "Is there any American out there who thinks Congress is too ethical?"
"Donald Trump and the Republicans said they're going to drain the swamp, and the very first thing they do is dismantle our outside ethics watchdog."
House Republicans voted 119-74 Monday in a closed-door meeting to gut Congress' independent watchdog. The full House will vote on the rules later Tuesday.
Virginia Rep. Bob Goodlatte's proposal would place the independent Office of Congressional Ethics under the control of lawmakers, which outraged Democrats and outside ethics organizations.
Republicans argued that the ethics office was too powerful, aggressive and overzealous and at times took away individuals' due process.
But Moulton doubted that those were the real motivations behind the GOP's decision.
"I guess what they have on their agenda might run into trouble with ethics. I don't know what else could be the reason," he said.
Despite Republicans being in the majority, Moulton is optimistic that he will be able to find common ground with conservatives who value accountability.
"The good news on things like this is that there are Republicans who oppose it," he said. "Even Speaker Paul Ryan opposes this change."
Rep. Dan Kildee, a Michigan Democrat, said Republicans had campaigned on planning to close the ethics office, the election outcome may have looked different.
"It was dishonest of them to do this in the middle of the night and to try bring it up without any debate at all. And unfortunately it looks like this will probably pass."
But the Michigan Democrat said his party will not allow this to move forward without a fight.
"This is not over. We're not going to just say, 'Oh, we lost this one,'" he said. "The American people are going to weigh in on this and they may have to make a change."
But Republican Rep. Marsha Blackburn said that the vote doesn't close the office entirely.
"It doesn't kill the Office of Congressional Ethics," she told Camerota. "What it does is to bring it under the jurisdiction or the oversight, if you will, (of) the Committee on Ethics."
"It provides individuals with the opportunity to have some due recourse and to know who is accusing them, which is not something that individuals have known," the Tennessee lawmaker said.
Blackburn said she hopes the move will make ethics investigations more open.
"My hope is what you will see is more transparency," Blackburn added. "You will see individuals with the opportunity to know who is accusing them and we will see matters resolved in a more timely manner."
The vice chair of Trump's transition team dismissed the idea that the decision was made "in the middle of the night."
"This is the time that the conferences meet," Blackburn said. "It's a matter of timing."
"Most people would say that five in the afternoon is not at night," she said. "And the Democrats have their meetings behind closed doors also."