The Rat Pack was the nickname given to a group of high-profile Canadian Liberal opposition Members of Parliament during the Progressive Conservative government of Brian Mulroney. They included Don Boudria, Brian Tobin, Sheila Copps, and John Nunziata.

The Liberal Party had been badly defeated in the 1984 Canadian election and many of its leaders were discredited or highly unpopular. In the first years of the Mulroney government the Liberals failed to mount an effective opposition to Mulroney, and were being overshadowed by the NDP, which had almost as many seats as the Liberals.

The group of young, energetic, and unknown Liberals soon emerged as a vocal voice of opposition to Mulroney. Mostly coming from the left flank of the Liberal Party they launched continued attacks on Mulroney in the House of Commons and the media.

When the Liberals formed a government after the 1993 election most of the Rat Pack members were given powerful cabinet posts, with the exception of John Nunziata who, after a number of high-profile disagreements with Jean Chrétien (including voting against the budget), was expelled from the caucus. He would later run - and win - as an independant candidate.

With that exception the Rat Pack fared well under Jean Chrétien, but under the somewhat more right of centre Paul Martin the remaining Rat Packers Sheila Copps and Don Boudria were relegated to the backbenches. Copps was then withdrawn from the caucus, when Tony Valeri was appointed the Liberal candidate for the newly - created riding of Hamilton East-Stoney Creek.