As harbingers and breakthroughs

Though governing parties are generally expected to fare less well in by-elections than in general elections, the swing in party support in a by-election compared to the previous election is often taken as an indicator of changes in general party support. A pronounced negative swing against a party over several by-elections may add to pressure for a change in party policy or even a change in leader, particularly when an opposition party does worse than expected or when the swing against a governing party is so pronounced as to suggest its impending defeat in the next general election. By-election upsets can also have a psychological impact by creating a sense of momentum for one party or a sense of impending defeat for a government. Deborah Grey's 1989 by-election victory in Beaver River was seen as evidence that the newly formed Reform Party of Canada would be a serious political contender and that it posed a serious political threat for the ruling Progressive Conservatives. It also provided important momentum for the new party. Similarly, the upset 1960 by-election victory of Walter Pitman in Peterborough as a "New Party" candidate was seen as a significant boost for the movement to replace the Co-operative Commonwealth Federation with an unnamed "New Party" which would be integrated with the labour movement. Pitman's candidacy in a riding in which the CCF was traditionally weak was seen as a test of this concept and his upset victory was used to convince the CCF and the labour movement to proceed with the founding of the New Democratic Party of Canada.

By-elections may occur singly, or in small bunches, especially if the authority responsible for calling them has discretion over the timing and can procrastinate. They are sometimes bunched to save money as holding multiple by-elections is likely to cost more than holding a by-election to fill the vacancies all at once. In Canada, in 1978, 15 by-elections were held on a single date, restoring the House of Commons to 264 members. The media called it a "mini-election", a test of the Liberal government's popularity with a general election due in less than a year. The 15 districts stretched from Newfoundland to British Columbia, and produced some unexpected results, for example, an NDP candidate winning in Newfoundland for the first time.

Party leaders and media commentators often point to by-election victories as important signals, but very often by-elections hinge far more on local issues and the charisma of the candidates (especially under single-seat constituency systems) than on national issues or how the voters feel about the governing party. Nonetheless it can be shown historically that a main opposition party which performs consistently poorly in by-elections is unlikely to be a serious contender for power at the subsequent general election.

New blood

By-elections may be used as a means for a political party, particularly a governing party, to bring star candidates into parliament. In Commonwealth parliamentary systems it is generally constitutional convention that cabinet ministers should have a seat in parliament, particularly in the lower house. Governments have, from time to time, appointed Cabinet ministers who do not have seats in parliament with the understanding that they will seek a seat as soon as possible. If no vacancy exists due to a death or resignation, political parties will sometimes persuade a sitting MP to give up their seat in order to allow a cabinet minister (or prospective minister) an opportunity to contest a by-election. Similarly, political parties whether or not they are in government may also persuade a sitting MP to step aside if the party leader or another leading party member is in need for a seat. Parties may also use a vacancy that occurred without such persuasion as an opportunity to seek out star candidates in order to bolster their front bench.

Usually, it is an MP in a safe seat that is persuaded to step aside. Consequently, an upset victory by the non-incumbent party in such a by-election can have a serious impact such as the loss of a cabinet minister or the loss of a party leader.

Upsets

In Canada, the most recent example of a cabinet minister appointed from outside of parliament having to resign after losing a by-election was in 1975 when Minister of Communications Pierre Juneau was appointed to Pierre Trudeau's Liberal cabinet directly from the private sector and tried to enter parliament through a by-election in Hochelaga. Juneau was upset by the Conservative candidate and resigned from cabinet ten days after his by-election defeat. General Andrew McNaughton was appointed to Cabinet as Minister of Defence on November 1, 1944 without having a seat in parliament after his predecessor resigned during the Conscription Crisis of 1944. A by-election was arranged in Grey North which the opposition Progressive Conservative party contested. The major campaign issue became the government's policy of "limited conscription" during World War II which McNaughton supported and which the Conservatives counterposed with a call for "full conscription". McNaughton was upset in the February 5, 1945 by-election. As a result, with confidence in his government undermined, Prime Minister William Lyon Mackenzie King called the 1945 federal election several weeks later when he had originally intended to wait until after the end of the war. McNaughton sought a seat in the federal election and resigned after he was again defeated.

In 1942, new Conservative Party leader Arthur Meighen sought to enter the Canadian House of Commons through a by-election in York South. His surprise defeat at the hand of Joseph Noseworthy of the Co-operative Commonwealth Federation ended his political career, and may also have been a factor in the Conservative Party's decision to move to the left and rebrand itself the Progressive Conservative Party under Meighen's replacement. Noseworthy's victory was also a significant breakthrough for the CCF giving it credibility as a national party where it has previously been seen as a Western Canadian regional protest party.

In the Canadian province of Ontario, John Tory, leader of the Progressive Conservative Party of Ontario ran in a 2009 by-election in Haliburton—Kawartha Lakes—Brock, after he convinced one of his caucus members to step down, in hopes of re-entering the Ontario legislature. His by-election defeat resulted in his resignation as party leader.

A Massachusetts special Senate election held in January 2010 produced a significant upset when Republican Scott Brown won the United States Senate seat formerly held for 48 years by Democratic Party stalwart Ted Kennedy. Republicans term the result the "Massachusetts Miracle" and argue that it is a harbinger of a revival in the party's fortunes.^[1][2][3]