With Congress always looking for new ways to boost tax receipts and protect individual investors, it’s natural for hedge fund managers to worry they have a bull’seye on their chests — especially now that word is out that some of them made more than $1 billion apiece last year.

Politicians of both parties have long criticized the lack of regulation of hedge funds, vast pools of capital that operate secretively, without having to make the disclosures that other investment firms such as mutual funds do. Adding to their explosive growth and unbridled operations, the jaw-dropping compensation of their executives has made hedge funds even more tempting targets for federal lawmakers.

That helps explain a recent surge in the hiring of lobbyists and stepped-up contributions to political action committees by managers in the trillion-dollar hedge fund industry and top officials of private-equity groups that have piled up billions in profits in recent years.

Hedge fund executives gave at least $2.3 million in campaign donations during the 2004 election, compared with $576,000 four years earlier, according to federal election data compiled by the nonpartisan Center for Responsive Politics. In 2006, that jumped to around $6 million.

“Particular industries reach a point where they get big enough that they realize that Congress can, intentionally or unintentionally, harm them through taxation or regulation,” a prominent Washington figure and lobbyist who headed the Republican National Committee in the 1980s, Frank Fahrenkopf, said.

Ever since a Fortune magazine writer and four pals scraped together $100,000 in 1949 to start what became the first hedge fund, the industry has flown under the public-policy radar. But the “absolutely obscene” profits, as one fund’s managing partner called them recently, and the massive loss to a California public pension fund from a hedge fund investment blowup last fall have drawn new and unwanted attention.

Senator Grassley of Iowa, the senior Republican on the tax-writing Senate Finance Committee, and Senator Baucus, a Democrat who heads the panel, are looking closely at how the profits of managers of hedge funds, as well as private-equity funds, are taxed, and whether that should be changed.

One question on the table at a closed-door meeting committee aides held this week with tax practitioners and academic experts: Is it fair for the managers’s portion of the funds’s anticipated future profits to be taxed at 15%, the rate for capital gains, rather than at income-tax rates of up to 35%?

Hedge funds swoop in and out of markets like day traders, investing millions in complex derivative securities and assets ranging far beyond stocks and bonds. Traditionally the domain of wealthy individuals, they increasingly have gained access to money from pension funds, mutual funds, and university endowments — meaning millions of people unwittingly now invest in hedge funds. They are believed to account for around 20% of all American stock trading.

Critics see a financial house of cards and point to two spectacular blowups: The 1998 collapse of hedge fund Long-Term Capital Management, amid the Asian financial crisis, rocked Wall Street and prompted the Federal Reserve to help arrange a $3.6 billion private bailout. And last September, fund Amaranth Advisors lost $6 billion because of wrong bets on natural gas prices, socking the employee pension fund of California’s San Diego County for an estimated $85 million.