Senators leaving Congress at the end of this year currently hold $5,334,251 in their campaign accounts and leadership PACs.
Campaign committees may use the funds for any lawful purpose, but the funds cannot be converted to personal use. Some members donate the funds to non-profits, charities, educational institutions, political parties, other candidates (within limits), or refund contributions to the original donors. Or they could leave the funds in the campaign account, and keep it going for years.
Leadership PACs of members of Congress are not restricted as much as campaign committees. There is no prohibition on the use of leadership PAC funds for personal use. The Federal Election Commission has recommended that the prohibition of the personal use of campaign funds be extended to apply to all political committees, including leadership PACs.
Here are the January 1, 2014, cash on hand figures for those Senators leaving at the end of this year: