The Rat Pack was the nickname given to a group of young, high-profile Canadian Liberal opposition Members of Parliament during the Progressive Conservative government of Prime Minister Brian Mulroney.

In the 1984 federal election, the Liberal Party had suffered the worst defeat in its history to date. In the first years of the Mulroney government, which took over the 33rd Canadian Parliament, the Liberals failed to mount an effective opposition to Mulroney and were in danger of being overshadowed by the New Democratic Party (NDP), which had won almost as many seats as the Liberals.

In the midst of this, a group of young, energetic, and hitherto little-known Liberals soon emerged as a vocal voice of opposition to Mulroney. They included Don Boudria from eastern Ontario, Sheila Copps from Hamilton, Brian Tobin from Newfoundland, and John Nunziata from the Greater Toronto Area. Tobin had been elected when the Liberals under former prime minister Pierre Trudeau regained power from the Joe Clark-led Conservatives in the 1980 election; the others had retained constituencies that retiring Liberal MPs elected in 1980 had won, and were seen to be among the few bright spots for the Liberals in the 1984 blowout. The Globe and Mail attributed the name "Rat Pack" to "a newspaper reporter" who was unnamed.^[1] They played roles in ending the careers of several of Mulroney's ministers due to their vocal criticism.^[2] The Rat Packers continued to serve as Official Opposition MPs after the PCs won the 1988 election and moved into supporting the government when the Liberals won in 1993.

Boudria, Tobin, and Copps were all appointed to Cabinet at various times during the subsequent ministry of Jean Chrétien.

Nunziata was expelled from the Liberal Party in 1996 for voting against the budget. He then ran—and won—as an independent candidate in the 1997 federal election and then lost to Liberal Alan Tonks in 2000. Tobin left Parliament in 1996 to become leader of the Newfoundland Liberal Party; he served as that province's premier until returning to Parliament in 2000.

Under Paul Martin, who succeeded Chrétien in 2003, Copps and Boudria were relegated to the Liberal backbenches. Tobin had already left Parliament in 2002 in protest of the stranglehold on the Liberal organization by Martin's supporters. In 2004, Copps' riding was merged with that of Martin loyalist Tony Valeri in such a way that most of the new riding's residents lived in Valeri's old riding. Copps was narrowly defeated for renomination. The last remaining member of the group, Boudria, had chosen to retire by the time of the 39th federal election in 2006.