CFU chairman David Matthew Jones said: "The situation facing the club on the pitch is very serious - off the pitch it is critical.
"The immediate challenge is to ensure Cambridge exists as a club on 1 March."
Fund-raising will begin with a bucket collection before Saturday's League Two game against Macclesfield at the Abbey Stadium.
£300,000 is needed by the end of the month to prevent the club going out of business, and another £300,000 by the end of the season.
The Us are bottom of the division with just 21 points from 31 games, seven points behind third-bottom Rushden and Diamonds.