Sen. Tim Johnson (D-S.D.) has maintained his characteristically low profile in the debate over financial regulatory reform, but colleagues of the man poised to take over the Banking, Housing and Urban Affairs Committee next year say he is actively engaged behind the scenes.

Senators and financial services industry sources say Johnson has been a sounding board for Chairman Chris Dodd (D-Conn.) as the reform package has taken shape, served as a liaison to moderate Democrats and helped shape key portions of the legislation.

With Dodd retiring this year, Johnson is slated to assume the Banking gavel in 2011 as long as the Democratic majority survives the November elections. Sources close to Johnson say his recovery from a December 2006 brain aneurysm continues apace and that he works as many if not more hours than most of his peers and is prepared to take over the committee.

“The Senator is ready and willing to fill any role that’s presented to him,” Johnson spokesman Jeff Gohringer said Tuesday.

The financial regulatory reform debate has centered on Dodd and ranking member Richard Shelby (R-Ala.), the legislation’s lead negotiators. First-term Sens. Bob Corker (R-Tenn.) and Mark Warner (D-Va.), both junior members of the Banking Committee, also have emerged as leading voices in the effort to overhaul Wall Street.

But Johnson, the second-ranking Democrat on the panel and the chairman of the Subcommittee on Financial Institutions, has said little publicly during a legislative process that has seen partisan divisions and three failed cloture votes before the official floor debate began late last week.

His subcommittee did not mark up any portion of Dodd’s bill, and he did not take the floor to speak on the bill until late Tuesday afternoon, shortly after his office was contacted for this story.

Sources familiar with Johnson say the third-term Senator and eight-year House veteran was never a show horse — including before his illness.

“A lot of what he’s been involved with hasn’t been front and center. That’s generally how he operates. He’s generally been a behind-the-scenes kind of guy,” a financial services industry lobbyist said. “To those who know him and follow Senate Banking, they would say this is how he works.”

Dodd said in brief remarks on the Senate floor Tuesday that Johnson’s help on the Wall Street overhaul bill has been “tremendously valuable.” Dodd spoke immediately after Johnson concluded his floor speech.

“I just want to say to my friend how much I appreciate his involvement and support over many months,” the chairman said. “He’s brought great value to this debate.”

Johnson hails from a Republican-leaning state, and his politics can be hard to pin down. He is at times a moderate, recently butting heads with Democratic Conference Vice Chairman Charles Schumer (N.Y.) over credit card reform legislation in an attempt to protect an industry that is a major employer in South Dakota.

But Johnson also has a populist streak: In 2008, he was one of just nine Senate Democrats who voted against the $700 billion rescue plan for the financial services industry. And unlike a host of centrist Democrats who hail from conservative Midwestern and Southern states, Johnson didn’t flinch in supporting his leadership on a series of controversial health care reform votes.

On financial regulatory reform, an issue on which he might have found himself at odds with his party, Johnson is on board with the majority as well.

“It has been discouraging to see some Members and special interests opposed to these changes. In fact, I believe it is hard to argue against these reforms with a straight face,” Johnson said during his floor remarks. “Yet those against reforming Wall Street have been doing just that, asserting that making markets fair and transparent will somehow hurt the economy. These reforms will help, not hurt, American consumers, small banks and small businesses.”

Johnson’s home-state colleague, Senate Republican Policy Committee Chairman John Thune, whom Johnson narrowly defeated in 2002, said that if Republicans fail to take control of the Senate, a Johnson chairmanship of the Banking Committee could be beneficial to South Dakota — this despite the fact that the two “don’t see eye to eye” on the current legislation.

“Financial services is such a big part of our economy. We’ll look forward to working with him,” said Thune, who won his seat in 2004 by defeating then-Majority Leader Tom Daschle (D) “He’s active on the issues. I know the community in South Dakota looks to him for direction on some of this stuff — and for help. And I hope on this bill he can provide some help.”

Warner confirmed that Johnson has been a point man in dealing with moderate Democrats on the financial overhaul, describing him as a “very strong Member, somebody I look forward to working with.”

And despite their difference on credit card legislation, Schumer also praised Johnson, saying he commands the respect of his colleagues and will be welcomed as the next Banking chairman. “He’s done a great job,” Schumer said. Tim has been a real player … on a whole lot of issues.”