In the quiet of their homes, gamblers are clicking their way ever deeper into debt.

Internet-based gambling -- highly controversial, the subject of pending legislation and illegal in many states -- is soaring, said Sebastian Sinclair, president of Christiansen Capital Advisors, a New York-based company that keeps gambling industry statistics.

Bets placed over the Internet are expected to total about $4 billion this year. That's about one-third more than last year, and the total is expected to quadruple over the next four years, he added.

Although all types of gambling are relatively controversial, Internet gambling is more so for two main reasons.

First, it's illegal in many states -- the only places consumers can gamble legally are licensed casinos and sports-betting operations that have been approved by the appropriate state government.

In addition, some experts say Internet games encourage compulsive gambling, an addictive behavior that can savage family finances.

To make matters worse, Internet gamblers often use credit cards to place their bets. Not only can that greatly increase the amount of money immediately available for gambling, losses racked up on losing bets come back to haunt the gambler at the end of the month -- and with an interest rate attached that can run 20% or higher.

Although all types of gambling can create debts, Internet gambling may cause people to fall further faster because it allows gamblers to feed their addiction quickly, privately and at any time of the day or night.

Both business and government have taken steps to curb Internet-based gambling. The House of Representatives this year passed an Internet gambling measure that would have barred banks from funding Internet bets. The measure failed to pass the Senate. However, industry experts expect the bill, or something similar, to be reintroduced next year.

This summer, New York Atty. Gen. Eliot Spitzer turned his attention to online gaming, securing an agreement from New York-based Citibank to block credit card sales from online casinos. Other credit card issuers, including MBNA Corp., Discover Financial Services and American Express Co., have followed suit.