New York City Mayor Bill de Blasio said Wednesday that he's "optimistic" about the politically progressive direction of Hillary Rodham Clinton's nascent presidential campaign but that he's still holding off an endorsement until he hears more.

Speaking on MSNBC's "Morning Joe" program, de Blasio said he and like-minded progressives plan to unveil next week in Washington, D.C., their for-the-left version of Newt Gingrich's "Contract With America" -- the platform that propelled Republicans to seize control of Congress in 1994.

The group aims to pressure candidates like Clinton to adopt its planks.

"I think she's beginning to fashion a progressive agenda," de Blasio said, citing her remarks this week supporting a path to citizenship for undocumented immigrants and those last week advocating reducing mass incarceration and adopting other changes to the criminal justice system.

De Blasio declined to endorse Clinton when she declared her candidacy last month -- a position that had not changed.

"I'm optimistic. I've seen some very powerful signs. But I think it's important to hear how she will address income inequality," de Blasio said. "That's what people are struggling with."

Called "The Progressive Agenda," the plan will be touted on the U.S. Capitol steps. Among the ideas: mandatory paid sick leave, progressive taxation, an increased federal minimum wage to $15 an hour -- it's now $7.25 -- and adopting the so-called Buffett rule, named after billionaire Warren Buffett, who has called for a minimum tax for the wealthiest Americans.

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The agenda's other planks include a national prekindergarten plan like the one de Blasio adopted for all 4-year-olds in New York City, and lowering the cost of college tuition, according to an op-ed he co-wrote with another progressive movement favorite, Sen. Elizabeth Warren (D-Mass.), in The Washington Post Wednesday.

De Blasio said he hopes to mirror Republican Gingrich's success two decades ago, only on liberal issues.

"It had a crystallizing effect for his party, conservatives. It was a clear, sharp set of ideas about how to change America -- in my view in the wrong direction," de Blasio said. "But as an organizational tool, it was very effective."

De Blasio said he also would hold a bipartisan presidential forum in the fall -- with candidates from both right and left invited.

Jumping into the fray, Gingrich, a former House speaker and Republican presidential contender, tweeted: "If Mayor DeBlasio comes up with a Contract from the left i would be glad to debate him about the two contracts- right and left."//VERBATIM AS TWEETED//