Moderate Republicanism, a tradition with roots going back to the party’s founding, officially died Tuesday, with the decision of Maine Senator Olympia Snowe to quit at the end of her current term.

Snowe was not a liberal, but nor was she a conservative. She occupied a reasoned middle ground that allowed her to work across lines of partisanship and ideology for decades. But on Tuesday she stole the thunder from the pathetic exercise that is the Republican presidential contest and made some really big political news.

In a stinging rebuke to her party and to the Congress, Snowe announced that she would forgo the prospect of re-election to a fourth term in a Senate that she argued is dysfunctional—and is not likely to improve.

“After 33 years in the Congress this was not an easy decision. My husband and I are in good health. We have laid an exceptionally strong foundation for the campaign, and I have no doubt I would have won re-election,” the 67-year-old Snowe announced Tuesday night.

“Unfortunately, I do not realistically expect the partisanship of recent years in the Senate to change over the short term. So at this stage of my tenure in public service, I have concluded that I am not prepared to commit myself to an additional six years in the Senate, which is what a fourth term would entail.”

Snowe is right that she would probably have been re-elected. Indeed, the greatest threat to the popular senator would have been in a GOP primary, not in the fall general election.

Snowe is also right that the Senate is a pit of petty partisanship, and that it is likely to remain so.

The senator had little potential to improve the circumstance because her brand of moderate Republicanism, once so influential, can now only seek compromises that leave no one satisfied. And with Snowe gone, moderate Republicanism will cease to be a meaningful force even for compromise. The senator’s supposed moderate allies, Maine Republican Susan Collins and Massachusetts Republican Scott Brown, are mere pretenders who lack Snowe’s statute and strength.

With Snowe gone, the center will not hold. Collins and Brown will disappear into the corporate abyss that is the modern Republican Party.

But that does not mean that the Senate will be harmed. While pundits and pontificators will weigh the meaning and import of Snowe’s exit for some time, the fact is that she could well be replaced by a dynamic senator with a record as an effective legislator and the potential to emerge as a progressive leader in the Senate.

Congresswoman Chellie Pingree, D-Maine, is considering a run to replace Snowe.

Pingree, the former president of Common Cause, is a serious progressive. As a Maine state legislator, she was a leader in the fight for healthcare reform. Now, Pingree’s an active member of the Congressional Progressive Caucus, who gets high marks—generally 100 percent ratings—from the AFL-CIO, NARAL Pro-Choice America and the League of Conservation Voters. She has also broken with the Obama administration, as is appropriate, on issues of war and peace, voting for the withdrawal of US troops from Afghanistan and explicitly opposing the deployment of US ground troops to Libya.

Pingree would go to the Senate as a savvy expert of a host of issues, including the campaign finance, media reform and net neutrality issues she worked on when she was with Common Cause. And she would be a likely leader on the left, with the potential—and the willingness—to stand beside Vermont Senator Bernie Sanders at the forefront of the fight for progressive policies and real reform.

Pingree, who came onto the national radar as a credible 2002 challenger to Republican Collins, is not in the race yet.

But she’s thinking about it, and she’s getting encouragement from groups such as the Progressive Change Campaign Committee.

Maine has a strong progressive bench. Several impressive candidates are already running, including State Senator Cynthia Dill and State Rep. Jon Hinck. Dill is a has been an outspoken critic of right-wing Governor Paul LePage, and won a key special election last year. Hinck, a co-founder of Greenpeace, USA, who represented commercial fisherman against Exxon-Valdez after the oil spill, and sponsored the first in the nation ban of BPA in plastics, has been an activist legislator. They could be joined in the race by former Governor John Baldacci and Congressman Mike Michaud, a great battler for American jobs as one of the leading congressional champions of fair-trade policies.

But if Pingree runs—and her announcement that she’ll be considering “how she can best serve Maine” suggests she may—she will be a striking presence in the race.

As a candidate, Pingree could join the list of progressive women who hold the real hope for transforming the Senate: Elizabeth Warren in Massachusetts, Tammy Baldwin in Wisconsin, Maize Hirano in Hawaii and Heidi Heitkamp in North Dakota.

All four are credible contenders with real prospects for securing wins that could play a critical role in helping Democrats retain control of the Senate. Chellie Pingree would run as the same sort of candidate, and she would ably fill the void created by Olumpia Snowe’s exit. Indeed, in a Senate that is divided along such stark partisan lines, Pingree could as an aggressive progressive with a reform message make more of a mark than Snowe has in recent years.