Washington (CNN) -- A key Senate Democrat came to the defense of embattled Nuclear Regulatory Commission Chairman Gregory Jaczko on Thursday after his four fellow commissioners sent a letter to the White House criticizing his leadership. They also testified before a House committee that Jaczko had "bullied" staff, restricted access to information and quietly assumed emergency powers following the Fukushima Daiichi nuclear disaster in Japan this year.
Sen. Barbara Boxer of California called a Wednesday House hearing a "witch hunt and an attempt to assassinate the character of a dedicated public servant," namely Jaczko. "Frankly, I was shocked, and I was appalled," she said.
Boxer charged that the other members of the commission were causing a distraction as a means to "slow-walk" the implementation of recommendations to increase the safety of U.S. nuclear plants after the Japanese accident in March.
Boxer's comments were directed at Wednesday's unusual hearing chaired by Rep. Darrell Issa, R-California, of the House Oversight and Reform Committee.
Jaczko's four fellow commissioners -- two Democrats and two Republicans -- accused him of creating a chilled work environment and intimidating commission employees. Jaczko was on hand and defended himself as passionate about his work to keep the nation's 104 nuclear power plants safe and said his words and actions had been "misconstrued" by employees.
On October 13, the four commissioners sent a letter to White House Chief of Staff William Daley saying that they had "grave concerns" about Jaczko's leadership and management practices, which they said had become "increasingly problematic and erratic."
Jaczko and the other commissioners appeared on Capitol Hill for the second day in a row Thursday, this time before Boxer's Senate Environment and Public Works hearing to review the progress in adopting a dozen recommended changes aimed at preventing something like the Fukushima nuclear accident from happening in the U.S.
Boxer said the chairman was the only Nuclear Regulatory Commission member aggressively pushing to make the recommended changes.
The other commissioners denied trying to slow progress and stressed that the issues were highly technical and required much review.
"I regret that these internal matters have been elevated to a public forum, and I accept my share of the responsibility for the situation," Jaczko said. But he denied charges that he was abusive to employees.
"I have never intentionally berated, threatened, bullied or intimidated any member of the staff," he said. "I can at times, as I've said, be passionate, be intense in my questioning. And if that has ever, ever led to an emotional reaction by somebody, I would want to know that, and I would address it immediately."
Commissioner William Ostendorff said he knew of three female commission executives who said they had been yelled at by Jaczko, and one of those felt she had been totally humiliated. But Ostendorff would not provide the names of the women.
Sen. Jeff Sessions, R-Alabama, said he thinks Jaczko has been abusive and has violated commission rules by denying other commissioners access to information.
Sen. Lamar Alexander, R-Tennessee, said he had worked with the chairman but is "deeply concerned" by the allegations against him.
Some senators urged the commission members to calm tensions and focus on their important work. Sen. Thomas Carper, D-Delaware, advised them to treat their colleagues the way they would like to be treated. He said he didn't think the commission had become dysfunctional, but he told the commissioners, "I don't think you're bringing your A game."
Boxer concluded the hearing by encouraging the commissioners and the chairman to "go out and celebrate the holidays together. I'll buy, honestly. Let's get on the same team. You are all smart."
CNN's Mike Ahlers contributed to this report.