The NDP intend to abolish the Senate entirely. They argue that they are an expensive ($100 million/year) and unelected body, with the power to block legislation passed by legitimately-elected members of Parliament. The Senate has also been plagued by numerous scandals in the recent past, involving corruption and bribery.
The major difficulty in getting rid of the Senate is that it requires unanimous approval from all 10 provinces, as well as support from the House of Commons and Senate itself. If unable to abolish the Senate, Tom Mulcair has the presumed support of up to 30 senators who would be willing to work with an NDP government to pass legislation.