Everybody's watching what's expected to be by far the most expensive presidential campaign in history and not without a dose of horror. Freed by the Supreme Court from spending limits, all manner of special interests are opening the spigots to buy influence.

But the bigger impact might lie elsewhere: down the ballot, where political races get less news media coverage and a few big ad buys can sway a race. Especially worrisome are secret donations, which are proliferating. A corrupting influence in any campaign, secret money is even more dangerous in less expensive races, where it can buy a seat in Congress or a state legislature, without voters knowing who the buyers are or what their agenda is.

Consider the 2010 race for Congress in South Dakota. Rep. Stephanie Herseth Sandlin, a rising Democratic star, and Republican challenger Kristi Noem each spent about $2.1 million to capture the state's lone seat in Congress. The American Action Network- a conservative group that isn't required to report donors - swooped in and spent more than $1 million, much of it in the campaign's final weeks to batter Herseth Sandlin with negative ads. In a presidential campaign, $1 million is chump change. In South Dakota, it's a game-changer: Herseth Sandlin lost by fewer than 7,200 votes. Shouldn't South Dakotans at least know where that money came from?

Of course they should. In its controversial 2010 Citizens United ruling, which opened the door for unlimited individual, corporate and union money to flow into politics, the Supreme Court recognised the importance of publicly naming donors, so "citizens can see whether elected officials are 'in the pocket' of so-called money interests."

Instead, the decision has fostered a whole new era of secrecy. Political strategists on both sides of the aisle have formed non-profit "social welfare" groups to escape disclosure requirements. They have names that convey selfless intentions, such as Americans for Prosperity, a conservative group, or the Patriot Majority USA, a liberal one.

These groups spent $137 million on the 2010 elections. Not surprisingly, one target was veteran Wisconsin Democratic Sen. Russ Feingold- a champion of campaign-finance reform - who lost his seat. Contributions from the Patriot Majority USA, meanwhile, helped save Harry Reid, the Senate's top Democrat, from defeat.

How big the impact will be this year is unknown, but guessing low would be unwise, particularly for lower offices. To put it in context, the average amount spent by candidates for Congress in 2010 was just more than $1 million ($1.4 million for winners; half as much for losers) - roughly equal to what American Action Network spent to defeat Herseth Sandlin.

Spending for state and local races is much lower, making them more vulnerable to big-money influence. The average cost of winning a seat in the state legislature in 2010, for instance, ranged from $711,000 in California to a remarkable $773 in New Hampshire.

Citizens United left the public only one way to protect itself from the rising threat: disclosure. At the federal level, this would be achieved by the Disclose Act, which we opposed two years ago because it exempted a number of powerful interest groups. Today's version, scheduled for Senate debate this month, requires that all groups - social welfare, union and business - to report all expenditures and all donations more than $10,000. Republicans, whose groups are drawing more money so far, are geared up to kill it.

In states, the situation is worse. A dozen states don't even require independent groups to report and state watchdog agencies are poorly funded.

The inevitable result is that come November, voters in many closely contested races will make their decisions based on a late flood of ads of dubious credibility paid for by people whose names and motives are unknown. How long it will take them to realise they're getting conned and demand disclosure is anyone's guess.

                                –USA Today editorial