WASHINGTON (AP) -- As the cost of presidential campaigns moves toward the $1 billion mark, leading congressional advocates of tougher campaign finance laws want to reduce the influence of special interests by pumping more taxpayer money into the system.

Three House members and one senator on Tuesday introduced legislation that would increase the amount of money in the presidential public financing system and increase the spending limits for candidates who participate in it. It would not change the rules for the 2008 presidential election.

"This system that's in place now is not sustainable," said Rep. Martin Meehan, D-Massachusetts, who introduced the legislation in the House with Reps. Christopher Shays, R-Connecticut, and David Price, D-North Carolina. Sen. Russ Feingold, D-Wisconsin, introduced similar legislation in the Senate.

The legislation would triple the amount available to candidates during state primaries and eliminate state spending limits. It would increase the amount available for general elections to $100 million (More than $80 million would be available in 2008). It also would offer more money to candidates whose opponents chose to decline the public money and raise more on their own.

To pay for the extra money, the sponsors would increase the voluntary presidential campaign checkoff on income tax returns from $3 to $10.

The proposals follow Democratic Sen. Hillary Rodham Clinton's decision to avoid the public system entirely during her 2008 presidential bid. Other candidates, including Sen. Barack Obama, D-Illinois, and Sen. John McCain, R-Arizona, are contemplating taking a similar step.