Horwath is again pushing her rivals to a debate on jobs and the economy, saying they're a priority for Ontarians in the June 12 election and deserve to be in the spotlight.
She had previously challenged her opponents to five debates, including one on northern Ontario issues.
The NDP says the Liberals have agreed to take part in the northern debate, which is planned for May 26 in Thunder Bay.
The Progressive Conservatives haven't said whether they'll show up, on top of participating in the scheduled leaders' debate on June 3.
But they've issued an open letter urging Premier Kathleen Wynne to participate in an all-candidates debate in her Toronto riding of Don Valley West.