SANTIAGO (Reuters) - A coalition of Chilean leftist parties on Thursday challenged presidential contender Alejandro Guillier to clarify his social and economic policies, withholding the outright endorsement sought by the center-left candidate.

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Beatriz Sanchez, the flagbearer for the hard-left Frente Amplio coalition, also criticized conservative ex-president Sebastian Pinera, the frontrunner, and called him a “step back” for the country, in a statement issued after a party meeting.

Pinera and Guillier will face off in a Dec. 17 runoff vote after placing first and second respectively in the first round of the election on Nov. 19.

Sanchez, who campaigned on a promise to tax mining companies and the “super-rich” in order to boost social spending, finished just two percentage points behind Guillier.

Her stronger-than-anticipated performance spooked markets and ensured the bloc will have influence, both in Congress and over campaigning for the second round.

“We are not the owners of anyone’s vote, so we call on our supporters to make their voices heard in this second round, and to vote according to their own convictions and analysis,” Sanchez said on national television.

She challenged Guillier to clarify his positions on her bloc’s priorities, including overhauling the country’s highly privatized pension system, forgiving the student loans of university graduates and re-writing Chile’s dictatorship-era constitution.

“This isn’t about negotiating with us. It’s about negotiating with those who overwhelmingly support these changes in our society,” she said.

Guillier did not immediately respond to the demands of the Frente Amplio. But winning over Sanchez’s 1.3 million voters is seen by Guillier’s camp as essential in triumphing over Pinera.

Guillier said at a campaign event on Monday that he “would need everyone’s support” to advance.

He has offered to forgive the debt of 40 percent of the country’s poorest university students and said he remained open to rewriting Chile’s constitution, both policies intended to appeal to leftist voters.

But Guillier must walk a fine line, as a too-sharp turn to the left could push more moderate voters - historically a large and potent voting bloc in Chile - to Pinera.

Both candidates would keep in place the top copper exporter’s longstanding free-market economic model, but Pinera has promised investor-friendly policies to turbocharge growth, while Guillier wants to press on with outgoing President Michelle Bachelet’s overhaul of education, taxes and labor laws.