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The Senate and the House of Representatives both have a government and an opposition team. The Prime Minister and the Leader of the Opposition are members of the House of Representatives and cannot participate in debates in the Senate—only senators can do this.
The teams in the senators need leaders too. The Leader of the Government in the Senate and the Leader of the Opposition in the Senate are each chosen by their party or parties to lead them in the Senate. While the leaders sit at the main table of the Senate, they do not have the same powers as the Prime Minister or the Leader of the Opposition; however, they do act as their main team spokesperson in the Senate.