By spending what is fast approaching a record $100 million on his re-election bid, Mayor Bloomberg is inadvertently raising doubts about New York’s campaign finance laws. The concept of a taxpayer-funded campaign finance system is now under scrutiny. Even some of Mr. Bloomberg’s supporters now worry he has put the system in jeopardy, while other New Yorkers question the entire notion of a taxpayer funded campaign finance system. If New York does decide to keep a system of low contribution limits supplemented by taxpayers’ matching funds, the system could benefit from adjustments.

Mr. Bloomberg is spending a fortune because that fortune offers him an advantage. Perhaps future rich candidates would spend less if that advantage went away. New York City could make the following adjustment: When a self-funded candidate ignores the spending limit set by the Campaign Finance Board (currently $11.5 million for mayor), the opponent would receive public money equal to whatever the big spender shells out over the limit.

A downside is that this could put taxpayers on the hook for a startling amount of money – that tab would have come to perhaps $90 million if such a rule had applied in the current race. But the mere risk of heaping a large bill onto taxpayers might convince rich candidates to hold back. They would certainly have less reason to exceed the campaign finance system threshold, knowing that every extra dollar of their own was another taxpayer dollar in their opponents’ pockets.

The issue here is not whether candidates should spend their own money, but how much they spend. Massive cash outlays threaten a successful campaign finance system that keeps corruption out of New York City elections. Candidates receive public matching funds in exchange for adhering to strict limitations both on how much they raise and how much they spend. This system prevents candidates from becoming beholden to donors, and also prevents the constant quest for campaign cash from drowning out discussion of important urban issues.

By tapping his own fortune, Mr. Bloomberg avoids the appearance of virtually any conflict of interest. And at number 40 on the Forbes 400 with an estimated net worth topping $5 billion, he certainly doesn’t need taxpayers’ help. But there is a growing concern that his campaign outlays make a mockery of the system, simply because the city’s campaign finance laws are ill-equipped to deal with spending levels appropriate for nationwide races.

The two most obvious ways to counteract this spending don’t work. One is unconstitutional and the other would defeat the very purpose of campaign finance laws. The first option would be restricting how much of their own money wealthy candidates could spend. The Supreme Court rejected this concept on First Amendment grounds. Or the system could allow individuals to give higher – or unlimited – amounts to the financial underdog, while also unshackling labor unions from strict rules that ban them from coordinating political activities with candidates. This would open the door to corruption or the appearance of corruption, the same twin devils of American politics the campaign finance system seeks to slay in the first place.

We can’t restrict rich candidates’ spending and we shouldn’t accept the risks associated with ending or altering contribution limits. But we can offer wealthy office-seekers an incentive to follow the city’s rules.

To be sure, Mr. Ferrer has hardly been his own best advocate. He’s raised very little money, even with the six-to-one matching funds on the first $250 of any contribution. But Mr. Ferrer is caught in a vicious cycle, where raising money becomes more difficult because Mr. Bloomberg seems unbeatable, and he seems unbeatable because he’s spending so much money.

Providing unlimited public money to candidates who face self-funded opponents is not a perfect solution. And any new system might need fundraising minimums to ensure public money isn’t wasted on candidates who aren’t viable. In fact, Mr. Ferrer’s lackluster fundraising record might disqualify him from such a matching system. But hopefully self-funded candidates would limit their spending voluntarily and the public would never have to shell out astronomical sums. Mr. Bloomberg would have extraordinary explaining to do if spending his own $100 million meant handing taxpayers a bill for nearly as much.

The premise of the campaign finance system is that New Yorkers are better off footing the bill than allowing money to hijack elections. The risk of increasing the tab for taxpayers is arguably preferable to leaving the electoral process vulnerable to the highest spender.

As Mayor Bloomberg’s spending helps pave his way towards a second term, the campaign finance system is buckling under the weight of his cash. In the future, perhaps candidates would decide spending beyond the system’s limits is futile if their opponents could match every extra dollar they spend.

Mr. Goldin is a host of NY1’s “Road to City Hall,” which airs weeknights at 7 p.m. and 10:30 p.m.