Former National Writers Union president Jonathan Tasini, one of the most outspoken progressive activists in the U.S. labor movement, is expected this week to launch a Democratic primary challenge to New York Senator Hillary Clinton on a progressive platform that features a call for bringing U.S. troops home from Iraq.

Tasini has scheduled an announcement for Tuesday morning in New York City, setting up a campaign that could put unexpected pressure from the left on Clinton, the unannounced frontrunner for the 2008 Democratic presidential nomination who until recently has been one of the strongest Democratic backers of the U.S. invasion and occupation of Iraq.

Tasini plans to campaign in support of the call by U.S. Representative John Murtha, D-Pennsylvania, for the rapid withdrawal of U.S. troops from that Middle Eastern country.

“Senator Clinton is out of step with the values of a majority of New Yorkers. While a majority of New Yorkers support an end to the war, Senator Clinton has repeatedly voiced her support for a war that continues to accumulate unacceptable costs, in terms of American and Iraqi lives and our own government spending,” explained Tasini, decribing a central theme of a campaign that is also expected to advocate for fair trade, economic reforms and universal health care.

Clinton has felt little heat so far from her most prominent Republican challenger, Westchester County District Attorney Jeanine Pirro, whose campaign so far has been so hapless that some top Republicans are now calling for her to quit the race and instead run for state Attorney General.

But Tasini, who served for more than a decade as head of a national union and has since worked as president of the Economic Future Group, poses a far different and potentially more interesting challenge to Clinton. An author and frequent guest on television public affairs programs, Tasini runs a well-regarded progressive blog, Working Life, at his website, where his reviews of trade, health care and labor policy issues have drawn a broad following.

Unlike Pirro, Tasini understands the issues, he’s quick on his feet, he knows his way around the state’s union halls and he recognizes that Clinton’s greatest vulnerability is a cautious centrism that has frequently put her at odds with grassroots Democrats.

Striking a chord that may well resonate with Democratic activists, Tasini says, “My candidacy will borrow a phrase from the late Senator Paul Wellstone, asking New Yorkers to’vote for what you believe in.'”

Even in liberal New York, a Tasini win in next September’s Democratic primary would be a huge upset.

Clinton has a deep-pockets campaign treasury, a solid Senate record and an appeal to many Democrats who see her as both an heir to her husband Bill Clinton’s legacy and potentially the best candidate to carry that legacy forward as a 2008 presidential contender. She also has an approach to even the most critical issues of the day that might charitably be referred to as “flexible.”

In 2002, Clinton broke with more progressive Democrats such as Wellstone, the late senator from Minnesota, Massachusetts Senator Ted Kennedy, West Virginia Senator Robert Byrd and Wisconsin Senator Russ Feingold, to support authorizing President Bush to use force in Iraq. And during the 2004 presidential campaign, she echoed the sentiments of the most hawkish Republicans when she criticized Bush for not sending enough troops to Iraq.

But, as the war has lost popular appeal, Clinton has begun to blur her position. In a November 30 letter to constituents, the senator seemed to back away from her support of the 2002 resolution, writing, “I voted for it on the basis of the evidence presented by the Administration, assurances they gave that they would first seek to resolve the issue of weapons of mass destruction peacefully through United Nations sponsored inspections, and the argument that the resolution was needed because Saddam Hussein never did anything to comply with his obligations that he was not forced to do. Their assurances turned out to be empty ones, as the Administration refused repeated requests from the U.N. inspectors to finish their work. And the ‘evidence’ of weapons of mass destruction and links to al Qaeda turned out to be false. Based on the information that we have today, Congress never would have been asked to give the President authority to use force against Iraq. And if Congress had been asked, based on what we know now, we never would have agreed, given the lack of a long-term plan, paltry international support, the proven absence of weapons of mass destruction, and the reallocation of troops and resources that might have been used in Afghanistan to eliminate Bin Laden and al Qaeda, and fully uproot the Taliban.”

Clinton stopped short of admitting that her 2002 vote was “wrong,” which is what former North Carolina Senator John Edwards, another prospective candidate for the 2008 Democratic presidential nomination, did in a recent Washington Post opinion piece.

She has also refused to side with another backer of the 2002 resolution, Murtha, who is now pushing for a quick exit strategy. Clinton claims that, “I do not believe that we should allow this to be an open-ended commitment without limits or end.” But, she adds, “Nor do I believe that we can or should pull out of Iraq immediately.” And a close read of her letter reveals that, while the senator is quick to criticize Bush, she is still in the camp that says America has “a big job to do” in Iraq.

That’s the opening that Tasini will attempt to exploit. It will not be easy — even some of his old allies in the labor movement will be slow to officially embrace his challenge to one of the most prominent and powerful Democrats in the country.

But frustration with Clinton runs deeper among activist Democrats than is often noted in the media.

Cindy Sheehan, the mother of a slain soldier in Iraq whose August protest outside George Bush’s ranchette in Crawford, Texas, made her one of the country’s most prominent anti-war advocates, has been almost as vocal in her criticism of the senator as she has been of the president. “Hillary Clinton is the leader of the pack” of pro-war Democrats, says Sheehan, who recently joined the board of the anti-war Progressive Democrats of America group. In an open letter posted in October on filmmaker Michael Moore’s web site, Sheehan wrote of Clinton: “I think she is a political animal who believes she has to be a war hawk to keep up with the big boys.”

Sheehan added that, “I will resist (Clinton’s) candidacy with every bit of my power and strength.”

That line led some New York activists to suggest that Sheehan should move to the state — as Clinton did before her 2000 Senate run — and run against the incumbent.

That’s not going to happen. Rather, Sheehan has issued a letter of support for Tasini’s challenge to Clinton, which you can read on Tasini’s website.