Few senators in either party have more steadily challenged the entrenched powers that be than Byron Dorgan, the North Dakota Democrats who traced his political roots to the proudly radical Non-Partisan League that challenged corporate power and big banks in the early years of the 20th century.

The loss of Dorgan, who on Tuesday made a surprise announcement that he would not seek a fourth term this year, is an exceptionally serious one for Democrats. The party will have a hard time holding Dorgan’s seat in a state that has not backed a Democrat for president since 1964.

But it is an even greater loss for progressives.

Unlike Connecticut Senator Chris Dodd, the other surprise Democratic retiree, the North Dakotan was never really comfortable with the insider politics of Washington.

Dorgan was one of the last of the old-school populists of the Upper Midwest, and he took his history seriously. In North Dakota, the Democratic Party is officially named the Democratic-Non-Partisan League Party — in recognition of the merger of the proudly radical NPL into the Democratic fold in 1956 — a fact that North Dakota’s senior senator never forgot. And never let anyone else forget.

In keeping with the NPL’s anti-corporate traditions, Dorgan was one of the few senators who fully understood the necessity of challenging economic royalists and business monopolies and duopolies.

It was not just his prescient opposition in 1999 to the repeal of the Glass-Steagall Act, which saw the North Dakotan decry the elimination of banking regulations by saying: “I want to sound a warning call today about this legislation. I think this legislation is just fundamentally terrible.”

On issue after issue, Dorgan challenged the consolidation of economic power in the hands of a few corporations.

He was especially outspoken on the issue of concentrated media ownership, emerging in recent years as one of the most determined congressional critics of big media.

But I will never forget Dorgan remarkable appearance at the “Rally for Rural America” that the late Senator Paul Wellstone, D-Minnesota, organized outside the U.S. Capitol in March, 2000.

There were plenty of outstanding speakers that day — including Wellstone — but no one captured the passionate populist mood so well as Dorgan. The senator from North Dakota loved ripped corporate agribusiness and the crowd — thick with members of the North Dakota Farmers Union — loved him for it.

We Dorgan raised the old NPL solidarity call — “We’ll Stick, We’ll Win” — the assembled farm families roared their echo to the senator’s cry. And it seemed for a moment at least that progressive populism was on the march once more.

Dorgan’s January, 2011, retirement will set him on a course of writing speaking, teaching and –undoubtedly — agitating.

He’ll remain a vibrant player in our poitical life — perhaps as a populist campaigner, perhaps as an Obama administration appointee working on farm and food issues.

But the Senate will be less for Dorgan’s absence. No senator in recent times — with the possible exception of South Carolina’s Fritz Hollings — was so able to channel the populist rage of William Jennings Bryan into the debates of the 21st century.