State Senate Majority Leader Joseph Bruno is rethinking the economic merits of the proposed stadium for the West Side of Manhattan. “If it’s a good business deal,” he said Wednesday, “it’s a good business deal for the private sector.” Mr. Bruno is at least half right.

A board controlled by Mr. Bruno, State Assembly Speaker Sheldon Silver, and Governor Pataki should vote soon on a $300 million state subsidy to the stadium, to add to the $300 million that city taxpayers will pay for the project. But while the city and state will spend equal amounts, the contributions aren’t equal on the merits. Mr. Bruno should follow his instinct and ensure that the state’s half of the money goes only toward justifiable public investment.

About half of the $600 million will pay for the construction of a heavy platform over the West Side rail yards, which are owned and will still be used by the Metropolitan Transportation Authority. The New York Jets will then pay to build their football stadium on top of that platform. The construction of basic infrastructure on public property to pave the way for a billion dollars in private investment might arguably be a smart use of public dollars (although a competing bidder said earlier this year that the land above the rail yards was so valuable that residential developers would pay for most of the cost of the platform itself).

But the other $300 million is more iffy. Mayor Bloomberg and Mr. Pataki propose that taxpayers spend about that much not on a vital piece of public infrastructure but on a speculative bet that should be made only by the private sector. The state and city would spend those millions to split the cost of a retractable roof over the stadium, to attract trade shows to a ready-made convention center when football’s not in season. An analysis by New York’s Independent Budget Office has found that taxpayers would earn a modest profit if the stadium could attract at least 14 annual conventions, including one annual “mega event.” This is a straightforward analysis – attract at least one trade show a month (plus two extra) and earn a profit for investors who would go in with the Jets to build a joint facility.

But why must taxpayers be unwillingly drafted as those investors? If tacking on hundreds of millions to a stadium means that facility stands a good chance of earning a profit as a convention center in the off-season, private investors would be eager to spend those millions themselves, in partnership with the Jets. A consortium of hotels, for example, could jointly raise the money for the roof, without a guarantee from taxpayers – sharing the risk within the tourism industry to increase the potential for hotel profits in the neighborhood. In fact, Mr. Bruno said he’s spoken recently with businesspeople interested in a deal with the private sector. “There are … investors who believe that the entire project can be funded with private investor money,” he said last week.

But a Jets spokeswoman told The New York Sun that the Jets haven’t spoken with these investors. The whole project is dependent on low-interest financing to be provided by taxpayers, she said – and private investors would never lend money at that same low interest rate. But she doesn’t want taxpayers to worry: The convention portion of the deal is still a “sound business investment.”

But those two statements are contradictory. Private lenders – those without a government guarantee – would demand a higher interest rate, and thus a higher return, on their own investment for a good reason: A convention center could be profitable, but staking cash on that outcome is still risky.

A company pays to send hundreds of employees to a convention only when that company is doing well, and lots of companies will do so only when the overall economy is sound. But when the economy is in recession, the convention center will likely do poorly – and taxpayers will be forced to cover its operating losses just when a future mayor and governor are struggling to close inevitable budget deficits. The mayor and governor would thus force taxpayers to take a long-term risk on a cyclical segment of the economy, for the chance of a tenuous reward: If all goes well, more tax dollars for the politicians to spend. The risk is even greater for taxpayers because there is a glut of convention-center space nationwide, a recent Brookings Institution report noted, and new space is unlikely to create or attract much new business anywhere.

If Mr. Bruno’s purported private investors find that a multimillion-dollar investment in a convention center is too risky for their tastes, he should thank them and realize that at least half of the proposed public investment in the overall project is too risky for regular taxpayers as well. Mr. Bruno could propose an easy compromise: State taxpayers will spend their half only for vital infrastructure on the West Side site – not make a bet on convention-industry profits.

Ms. Gelinas is a contributing editor of City Journal, from whose Web site, city-journal.org, this is adapted.