She acknowledged for the first time that some of her left-leaning party's traditional supporters were turned off by the NDP platform.
"We did have some things in our platform that we thought were responsive to some of your concerns," she said at the annual meeting of the Elementary Teachers' Federation of Ontario.
"I hear you clearly that you didn't feel it was enough and I certainly take that back as constructive criticism and something that I certainly will pay attention to."
The New Democrats kept 21 seats, but lost three important Toronto ridings to the Liberals.
Some high-profile party members spoke out against Horwath during the campaign, saying she'd lost her way with populist policies and by rejecting the Liberals' left-leaning budget, which triggered the election.
NDP strategists made no bones during the campaign that they were going after voters who had supported the Progressive Conservatives in the past, but didn't like then-leader Tim Hudak.
Part of the campaign was about reaching out to voters that they hadn't reached in the past, Horwath said. But the NDP's values and principles as a social-democratic party have "never wavered."
Some ETFO members told Horath they were disappointed with the NDP's campaign, saying they found the platform lacking.
"I would encourage you to really, if you really want your base to expand, that you really need to commit to concrete ideas which I unfortunately did not see in this past election," said one union member.
"I hear what you're saying and I hear the concern about the lack of clarity around some of the very basics that we've always talked about, but you couldn't find them this time around," Horwath said.
The NDP chief struck a different tone two months ago, when she said she had no regrets about the campaign and that voters didn't reject her party, but voted for the Liberals out of fear of Tory job cuts.
This time, Horwath offered a mea culpa.
"There's no doubt that we have some learning to do in terms of how we approached our campaign," said Horwath, who faces a leadership review in the fall.
"It's something that I take seriously and that I'm very concerned about."
Horwath pledged to hold the Liberals, who emerged from the election with a majority government, to account on education issues, such as abandoning standardized testing, finding new uses for unused schools and reducing class sizes.
The NDP will take the government to task for any gaps between what they promised and what they deliver, she said.
"I would submit to you: isn't there another gap between what you're saying today and what we heard — or rather what we didn't hear — during the election campaign," said another ETFO member.
Another union member asked Horwath how she can hold the government accountable when she triggered an election that robbed her party of any clout it had in a minority parliament.
ETFO, which represents about 76,000 education workers, is headed to the negotiation table with the Liberals and school boards as its current contracts expire Aug. 31.
Two years ago, the union and others went to war with the Liberals over imposed contracts that froze pay and limited their ability to strike.
But many public sector unions helped the Liberals defeat the Tories in the election over Hudak's disastrous promise to cut 100,000 public sector jobs to help slay Ontario's $12.5-billion deficit in two years.
Although the Liberals promised not to shrink the public sector, they have said that there's no new money for compensation as they try to balance the books in 2017-18.
ETFO president Sam Hammond said the Liberals don't owe the union anything.
"What we want to do is go into bargaining in a respectful way with all three parties and negotiate respectfully and try to find solutions to the items that are put on the table," he said.
Also on HuffPost