America’s decrepit and unpopular system of taxpayer financing of presidential campaigns has suddenly taken center stage in this year’s election — with Sens. John McCain and Barack Obama each caught between his long-expressed principles and his self-interest.

McCain and Obama have long boasted of their support for aggressive regulation of money in politics. But now each would love to avoid taking public money. In that irony lies a lesson about campaign-finance “reform.”

The presidential public-funding system dates to 1974, after President Richard Nixon resigned. At the time, Democrats held large majorities in Congress, but their presidential candidates faced serious funding challenges.

Since 1960, GOP presidential candidates had opened up a growing fund-raising lead over Democrats. If that trend had continued, Republican presidential candidates would be raising many times the sums raised by Democratic ones in 1976 and beyond. Public financing put an end to that threat.

The law offers equal sums to both major-party candidates for the fall campaign — so long as they don’t raise money from private contributors. That imposed an equality of funding, stopping the growing GOP edge.

In other words, for all the talk of “reform” and corruption, regulations of campaign finance in fact are all about manipulating elections to boost specific interests. (In ’74, it was Democrats in the presidential arena. Other “reform” laws have greatly benefited incumbents of both parties, at the expense of challengers. Still others, like recent New York City laws, boost labor unions’ political power at the expense of businesses’.)

Back to the ’08 race. Sen. McCain started his White House bid with private financing but ran out of cash last June. To keep going, he sought to qualify for public funding for the primary phase of the campaign — and used the possibility of such money as collateral for a loan.

That move got him through. McCain now is the sure GOP nominee. But it puts him in a bind as he looks toward the general election. If he’s trapped in the public-funding system, he can can spend only $4 million more between now and the Republican convention. That’s far too little to keep his message before the public.

And it’s a disaster for him. Barack Obama and Hillary Clinton have turned out to be extraordinary fund-raisers — and opted out of the public system for the primaries. If either captures the nomination soon, he or she can keep raising and spending private money all spring and summer long. McCain might well have lost the fall race before he receives more public money after the Republican convention.

Not surprising, McCain desperately wants out of the public system. (And since he hasn’t actually received any public funds yet, he may be able to escape.)

Obama, meanwhile, is eager to escape the public system for the general election, the phase that begins after the conventions. He promised last year to take public financing if the GOP nominee did — but he’s now learned that he could vastly outraise McCain in the private sector.

The American public plainly shares this cynical view of our supposely “clean” public-finance system. In recent years, only about 6 percent of federal tax filers have checked off the box that directs $3 from their payment to the presidential campaign fund.

That’s a 94–6 vote of no confidence in the system.

The system of public funding — and heavy regulation of private giving — for political candidates has its sincere defenders, people who believe democracy demands it.

But the facts speak for themselves. The system was enacted for partisan gain, not for lofty public purposes. And now two of its strongest supporters are looking to game the system — to escape public funding and rely on private cash — because it’s in their clear self-interest.

It’s time to end the wretched pretense that this system serves democracy.