The party again became the Official Opposition after the 1948 general election, and defeated the Conservative premier George Drew in his seat, when Bill Temple unexpectedly won in the High Park constituency.

Macphail later served in the Legislative Assembly of Ontario as the CCF Member of Provincial Parliament (MPP)[Note 1] for York East from 1943 to 1945 and again from 1948 to 1951.

[13] The party received 7.1 percent of the vote and, with Lawrence's election in Hamilton East, won its first seat in the Ontario Legislature.

Alderman John Mitchell of Hamilton was elected the first Ontario CCF president in 1934 as part of a reorganization of the party after its provincial council had been suspended by federal leader J. S. Woodsworth for suspected communist infiltration.

[15][16][17][18] Graham Spry, a publisher and broadcaster who was also a member of the LSR, served as the Ontario CCF's vice-president of its provincial council from 1934 to 1936.

[22] The newly created Leader position's role was political and legislative, while internal CCF affairs and administration would remain the president's domain.

[22] The party achieved a major breakthrough under Jolliffe, in the 1943 general election, when it formed the Official Opposition with 32 percent of the vote and 34 seats.

The CCF was just four seats short of George Drew's Progressive Conservatives ("Tories"), who formed a minority government that was the beginning of what became a 42-year political dynasty.

[24] Much of the source material for the anti-CCF campaign came from the Ontario Provincial Police (OPP)'s Special Investigation Branch's agent D-208: Captain William J.

Instead, starting in November 1943, he was investigating, almost exclusively, Ontario opposition MPPs, mainly focusing on the CCF caucus.

Jolliffe replied by giving a radio speech – written with the assistance of Lister Sinclair –[28] that accused Drew of running a political Gestapo in Ontario.

[29] It is my duty to tell you that Colonel Drew is maintaining in Ontario, at this very minute, a secret political police, a paid government spy organization, a Gestapo to try and keep himself in power.

And Col[onel] Drew maintains his secret political police at the expense of the taxpayers of Ontario – paid out of the Public Funds....[27]Jolliffe's inflammatory speech became the main issue of the campaign, and dominated coverage in the media for the rest of the election.

[31] Jolliffe's CCF and Mitchell Hepburn's Ontario Liberal Party wanted the election suspended until the commission tabled its report.

[34] A Gallup poll done a month earlier showed the CCF at essentially the same percentage, making it questionable whether or not the "Gestapo" speech had an effect on the campaign.

[35] Drew, with his attack campaign, successfully drove the voter turn-out up, thereby driving the CCF's percentage and seat totals down.

[39] Sanderson was, in late 1943 to 1945, along with Gladstone Murray, leading the libellous advertisement campaigns against the CCF in newspapers and billboards, with information gleaned from Dempster's briefings.

[43] Because of Lewis's discovery, Drew's son Edward, placed extremely restrictive conditions on his father's papers housed in the Public Archives of Canada.

[44][Note 2] As Lewis pointed out in his memoirs, "We found that Premier Drew and Gladstone Murray did not disclose all information to the Lebel Commission; indeed, they deliberately prevaricated throughout.

"[46] After the LeBel Report was published, the Ontario CCF still had to go on with the business of running the party, and hold its annual convention.

[47] His critics charged that the CCF did not stress policy enough during the election; that the party's platform was too vague, too leader-based, and was too reliant on slogans.

[47] The motion read, "the campaign tactics of the 1945 election had been decided by Mr. Jolliffe with the advice of Mr. Lewis and the democratic processes within the CCF had been ignored.

[47] On Saturday afternoon, after the grievances were aired, the convention unanimously passed a resolution condemning Premier Drew asking him to stop spying on labour and political officials.

[52] The convention was held at the Royal Connaught Hotel in Hamilton, Ontario from December 9–11, 1946, the city where the United Steelworkers of America (USWA) went through a long protracted strike about reducing the work-week to 40 hours.

[54] Professor Grube stepped down as the president, and Andrew Brewin succeeded him after defeating former York South MP, Joseph W. Noseworthy by four votes.

[54] The main resolution that would affect the upcoming provincial election was one that condemned Drew's government for its hastily approved legislation allowing for cocktail bars to operate in Ontario.

[54] Though the 1948 election came about a year sooner than normal, the Ontario CCF had been expecting this because of the polling information available that indicated that Drew's popularity was falling.

[60] The increasing role of the trade union leadership in the party was unpopular with some activists like MPP Bill Temple.

[60] The "Ginger Group" led by Temple, True Davidson and others was formed in the wake of the disastrous 1951 electoral result which they blamed on the "bureaucratization" of the party and its movement away from socialist principles and particularly socialist education, developments for which they held what they saw as the conservative, anti-democratic and bureaucratic influence of the OFL as responsible.

[63][64] The Ontario CCF Council ceased to exist on Sunday, 8 October 1961, when the newly elected NDP executive officially took over.