Horwath was elected to the
Legislative Assembly of Ontario in a 2004 by-election in the then-extant provincial
riding of
Hamilton East, defeating
Liberal candidate Ralph Agostino to succeed the deceased Liberal member
Dominic Agostino, Ralph's brother. Winning 63.6 per cent of the vote, up from the NDP's 29.4 per cent in that riding six months earlier, her landslide victory boosted the NDP's seat count over the threshold for
official party status in the legislature, and helped give the federal
New Democratic Party a bounce in Hamilton that would continue into the
federal election shortly thereafter. In the
2007 election, Horwath ran in the new riding of
Hamilton Centre, due to
redistricting that divided her former Hamilton East riding between Hamilton Centre and the new riding of
Hamilton East—Stoney Creek. Horwath's new Hamilton Centre riding included approximately half of her former riding as well as a portion of the former Hamilton West riding where she had run federally in 1997. It also included her entire former city council ward. In the lead up to the campaign, Horwath was expected to face Hamilton West Liberal incumbent
Judy Marsales. However, Marsales opted not to run for another term, and Horwath easily defeated Liberal candidate Steve Ruddick on election day.
Leader of the Ontario NDP 2009 leadership election On November 7, 2008, Horwath officially launched her campaign to win the party's leadership. The
leadership election was held March 6–8, 2009. Horwath led on the first two ballots, and won on the third ballot with 60.4% of the vote defeating
Peter Tabuns,
Gilles Bisson and
Michael Prue.
2011 general election The
2011 provincial election saw a rise in support for the NDP under Horwath's leadership. The party won more than 20% of the popular vote for the first time since
1995 and almost doubled its seats to elect 17 members of the legislature. The election also resulted in the Liberal government of
Dalton McGuinty being reduced to a
minority government with the NDP holding the balance of power. In April 2012, Horwath passed a
leadership review at the party's convention with 76% support.
2014 general election On May 4, Horwath announced that the NDP would be voting against the Liberals' proposed budget, triggering a spring election. In the
2014 provincial election, the NDP was able to maintain its seat count of 21 at dissolution despite the loss of three seats in Toronto, but lost the balance of power when the Liberals took a majority win in the election. Horwath has faced criticism from some party members and progressives for running a
populist campaign which they described as right-wing. Despite criticism of her leadership from some quarters, Horwath received a slightly increased level of support, 77%, at the party's post-election convention held on November 15.
2018 general election Horwath ran in her
third election as NDP leader against the Liberal government led by
Kathleen Wynne and a Progressive Conservative Party led by
Doug Ford. Horwath promised to introduce "Canada's first universal Pharmacare plan", highlighted by a universal
dental plan and a
prescription drug plan that "will initially cover 125 of the most commonly prescribed drugs". She also promised a
child care plan in which seventy per cent of Ontario parents "would either have free child care or pay an average of $12 a day in a licensed not-for-profit daycare". She also said that she would close the
Pickering Nuclear Generating Station immediately, while the other party leaders have pledged to keep it open until 2024. The NDP promised to increase corporate tax rates from 11.5 to 12.5 per cent, as well as introducing an income tax increase for those earning over $220,000 per year. Horwath said the province would fund half of the operating cost of municipal transit and indicated that she would not introduce back-to-work legislation. leading to greater media attention and greater scrutiny. With her party gaining
official opposition status, she became the
Leader of the Official Opposition during the
42nd Parliament, the second highest number of seats in the party's history. The NDP took all of old Toronto (i.e., what was the city of Toronto before the 1998 creation of the "megacity" of Toronto), as well as all but one seat in Hamilton and all but one seat in Niagara.
2022 general election Horwath and the NDP released their 2022 platform in April 2022. The NDP's campaign focused on increased funding for social programs and government services, which would be paid for through higher taxes on businesses and individuals earning over $200,000 per year. Funding would go toward reducing class sizes, raising welfare payments and disability payments, subsidies for black, indigenous and LGBTQ+ entrepreneurs, hiring more healthcare and education staff and increased wages for public servants. The NDP also proposed to expand COVID-19 vaccine mandates, implement a mixed member proportional electoral system, to close down all privately owned long-term care facilities and to stop the construction of new highway projects. Horwath was re-elected in Hamilton Centre and the NDP won opposition, but it lost 9 seats and placed third in the popular vote. Horwath resigned as leader election night. The election set a record for the lowest voter turnout in an Ontario provincial election, as only 44.06% of the people who were eligible voted. On August 15, 2022, she resigned as the MPP for Hamilton Centre. ==Mayor of Hamilton (2022–present)==