In bald terms, Wynne warned NDP supporters and other progressives that a vote for Horwath would be a vote for Progressive Conservative Leader Tim Hudak.
"You can't stop Tim Hudak by voting NDP in this election," Wynne told a crowd in Mississauga.
"If Tim Hudak has half a chance, he is going to destroy so much of what we have built up over decades."
Speaking to an enthusiastic group of supporters and fellow candidates, Wynne warned repeatedly — as she has done in recent weeks — that a Hudak government would be dangerous for the province.
But while Wynne's anti-Hudak rhetoric was familiar, her direct pitch to New Democrats — whom she has largely ignored through the campaign — was telling of a vote too close to call.
Horwath, she noted, has refused to rule out supporting Hudak in a coalition government — something Wynne said iconic NDP leaders would never have contemplated.
"That is how far the NDP has fallen — it's not the party that it was," Wynne said. "It's not the party of Jack Layton. It's not the party of Ed Broadbent. It's not the party of Stephen Lewis."
Horwath quickly dismissed Wynne's suggestion that Ontarians should vote strategically.
"I think Ontarians are in a position to know that you don't have to listen to someone who tells you how to vote and tells you how to think," she said Sunday as she ramped up her campaign trail rhetoric.
"You don't have to pick between corrupt and crazy."
The NDP leader also suggested Wynne was trying to mislead voters in an op-ed the Liberal leader penned for the Toronto Star on Sunday which urged NDP supporters to opt for the Liberals.
"She wants people to believe that Hudak is a factor in Toronto. Hudak is not a factor in Toronto. What Ms. Wynne actually fears though, is she fears NDP MPPs," Horwath said. "I think Ms. Wynne is trying to scare people's votes."
Most polls in recent days have pointed to a tight race between the Liberals and Hudak, who is pledging to create one million jobs over eight years by cutting government and slashing 100,000 public-sector employees. Hudak has admitted, however, that about half of those jobs would be created through normal economic growth.
The polls also suggest the NDP — third party in the last legislature — are running well behind the other two.
Horwath, whose party had propped up the minority Liberals, triggered the election by voting against the Wynne budget at the beginning of May on the basis that the scandal-riven party could not be trusted to implement its pledges.
In an effort to win support, the NDP has moved to the right of its traditional position in this election, making several pocketbook pledges, such as cutting the provincial portion of the HST on hydro bills and lowering auto insurance rates.
Wynne, however, evoked Toronto Mayor Rob Ford as she blasted the New Democrat platform as an "incoherent list of disconnected ideas that tap into a populist oversimplification of what public policy is."
The NDP, she said, came up with a "Rob Ford-like-style" platform, a reference to the mayor's approach.
By contrast, Wynne said, the Liberals had put forward the most progressive budget in decades, something the NDP shot down.
Hudak, she said, would return Ontario to the days of former Conservative premier Mike Harris, whose tenure in the mid-1990s was marred by teacher strikes, labour unrest and anti-poverty violence.
"We all know that this province suffered when Mike Harris was the premier. We know the kind of havoc and chaos that he created," Wynne said.
"Tim Hudak wants to go farther than Mike Harris."
At his own campaign event on Sunday, Hudak warned that the Liberals would be trying to frighten people about the Conservative plan.
That's because the Liberals, who are "addicted to power," Hudak said, don't want to talk about their record.
"We've seen a Liberal party leader in Kathleen Wynne who spent the entire campaign attacking everybody else and telling you what not to vote for," Hudak said.
Ontario goes to the polls on June 12.
— with files from Allison Jones and Will Campbell.