SANTIAGO (Reuters) - The leader of an influential leftist bloc in Chile endorsed center-left presidential hopeful Alejandro Guillier on Monday over conservative Sebastian Pinera in next week’s run-off election.

Chilean presidential candidate Alejandro Guillier speaks to supporters after the results of the first round vote during the presidential elections in Santiago, Chile, November 19, 2017. REUTERS/Pablo Sanhueza

Beatriz Sanchez, the flagbearer for the hard-left Frente Amplio coalition, said Pinera’s suggestion that ballots had been tampered with in the first-round election had changed her mind about staying quiet on whom she would vote for in the run-off.

“That crosses the line and that’s why today I’ve decided...to vote against Sebastian Pinera,” Sanchez told journalists. “My vote will be for Alejandro Guillier.”

Winning over Sanchez voters has been seen as essential for a Guillier triumph over Pinera in the second-round vote. As Frente Amplio’s presidential candidate, Sanchez secured twice as many votes as expected by opinion polls and came two points short of moving onto the run-off election with Pinera.

But it was unclear if Sanchez’ somewhat reluctant endorsement of Guillier would be enough to get her supporters excited about heading to polls Dec. 17. Last week, Frente Amplio refrained from endorsing Guillier and demanded he clarify his proposals.

Pinera, a former president who governed Chile between 2010 and 2014, had been expected to easily win this year’s election before his disappointing performance in the Nov. 19 first-round vote.

On Monday, Pinera said on a local radio program that some voters had reported that ballots were pre-marked in favor of his rivals in the first-round election and that he would have more supporters supervising voting stations in the run-off vote.

Chile’s electoral authority said it had received no complaints of irregularities and Pinera’s remarks were widely criticized.

“Let’s be responsible and not discredit our democratic institutions,” outgoing center-left President Michelle Bachelet said on Twitter.

Pinera said in an impromptu news conference later on Monday that he did not mean to cast doubt over election results but reiterated that he believed ballots had been tampered with.