Money in politics has often been described as “like squeezing a balloon” —that is, the air/water gets displaced within the balloon but doesn’t leave it. The political adage has been cited in print since at least 1996.

A similar saying is “Money in politics is like water flowing downhill”—in other words, an unstoppable force of nature’s gravity.
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29 December 1996, Rocky Mountain News (Denver, CO), ‘Political will finds a way: Campaign reform won’t stop money from flowing” by Peter Blake:
Trying to limit money in politics is like trying to push down on a balloon. When one area goes down, another pops up.
 
3 April 1997, NPR: Morning Edition, “Campaign Contributions”:
But despite efforts to limit the influence of money in politics, some say the new laws force candidates to spend even more time and energy fundraising (...) It’s kind of like squeezing a water balloon. You try to take the money out of one area of the political system and it reappears in another.
   
Google Books
U.S. News & World Report
v. 122 - 1997
Pg. 110:
Squeeze out soft money, this argument goes, and the money-in-politics balloon would merely bulge elsewhere.
   
27 January 2000, Albuquerque (NM) Journal, “Campaign Reformers Have Hopes Up” BY Barry Massey (Associated Press):
“There is a concern that money in politics works like a water balloon. If you squeeze it one place, it sort of pops out in another place,” says Amelia Myer of Common Cause in New Mexico.
   
24 March 2001, Los Angeles (CA) Times, “The Sound of Cha-Ching! Still Music to Reformers’ Ears” by Greg Miller, pg. A12:
Campaign finance reform “is like a water balloon,” said Bob Rusbuldt.
 
Google Books
Money, politics, and campaign finance reform law in the states
By David Andrew Schultz
Durham, NC: Carolina Academic Press
2002
Pg. 22:
The moral from state experiences can be used to confirm an old political adage that regulating money in politics is like a water balloon: if one squeezes one end of the ballon, the water will shift elsewhere.
     
Minnesota Public Radio
New 527 groups enter the political debate
by Laura McCallum, Minnesota Public Radio
July 25, 2004
(...)
“There’s that old metaphor that money in politics is like a water balloon—you squeeze it one place and it goes somewhere else,” Schultz said.
 
Bloomberg.com
Congress May Push Donors to Unregulated U.S. Political Groups
By Kristin Jensen and Michael Forsythe
July 11, 2005 (Bloomberg)
(...)
“Money in politics is like air in a balloon: Any time you compress it one place it’s going to pop out somewhere else,” says Kenneth Gross, who was the Federal Election Commission’s head of enforcement in the 1980s.
 
Google Books
Winning Elections with Political Marketing
By Philip Davies and Bruce I. Newman
Routledge
2006
Pg. 224:
As David Schultz concluded from a twelve-state examination of campaign finance reform experiences:
 
...regulating money in politics is like a waer balloon: if one squeezes one end of the balloon, the water will shift elsewhere. Regulations that seek to limit money in politics do not seem to prevent contributors from giving; they simply shift their donations to another venue.
     
MySpace
Bob Confer
Posted:  Jun 15, 2007 4:59 AM
From the Buffalo News…
Watson: Open market lets you buy a politician
Rod Watson
Updated: 06/14/07 8:57 AM
(...)
Critics have long complained that reforms to ban soft money, limit hard money and regulate campaign advertising are not only unconstitutional, they’re unworkable. They insist the campaign system is a giant balloon: Squeeze it in one place, and it expands someplace else.