The practice of lobbyists lavishing huge sums of money on charitable causes championed by Members of Congress and on events to honor lawmakers and other federal officials may be losing some of its appeal.

Corporations, labor unions and other professional groups spent $25.2 million on such events and causes in 2009 — a drop of $15 million, or 38 percent, from the previous year, the first time such reporting was required.

Experts cite a number of reasons for the steep decline, including the sluggish economy as well as changes in disclosure rules. They say lawmakers may be more reluctant to attend events underwritten by lobbyists who have business before Congress, particularly now that such contributions are publicly disclosed.

“People are looking over their shoulders,” said Steve Ellis, vice president of the watchdog group Taxpayers for Common Sense. He added that in the current economic and political climate, lawmakers may want “to appear a little less Marie Antoinettish.”

While federal laws limit the amount of money that individuals and groups can contribute to Congressional campaigns, there are no restrictions on donations to lawmakers’ favorite causes, charities and institutions. Congress approved new disclosure laws that went into effect in 2008 after a series of ethics controversies involving lobbyists contributing to lawmakers’ pet causes.

A number of major corporations slashed their contributions in 2009. The beleaguered Federal Home Loan Mortgage Corp., also known as Freddie Mac, made no contributions on behalf of lawmakers in 2009 compared with $639,011 the previous year. The engineering company Brookfield Atlantic Corp. also made no donations last year compared with $501,500 in 2008.

The Laborers’ International Union of North America’s contributions fell by 66 percent to $542,860 in 2009, and defense contractor Raytheon Co.’s donations dropped 77 percent to $172,276 last year.

The biotechnology company Amgen Inc. dramatically scaled back its contributions by 94 percent to $229,020. By contrast, in 2008, Amgen made a $5 million contribution on behalf of the late Sen. Edward Kennedy (D-Mass.), the former chairman of the Health, Education, Labor and Pensions Committee.

At least $3.7 million of the total decrease in honorary contributions for 2009 is connected to events honoring Kennedy, who died last year. The donations were largely for the Edward M. Kennedy Institute for the United States Senate in Boston.

Kennedy drew more contributions in his honor over the past two years, $9.1 million, than any other lawmaker or federal official. But most of that money — $6.4 million — came in 2008, while he was battling brain cancer.

Despite the overall drop in giving, many companies continued their contributions on behalf of lawmakers and their causes. Some large entities — including Ford Motor Co., EMC Corp., Walmart, General Mills Inc. and the American Federation of State, County and Municipal Employees — substantially increased their giving on behalf of lawmakers in 2009 compared with 2008.

The Congressional Black Caucus Foundation conference drew high-profile sponsors such as Exxon Mobil Corp., which gave $100,000.

Toyota Motor Corp., which is now facing Congressional investigations stemming from a massive vehicle recall, contributed just more than $510,000 on behalf of lawmakers last year, only a slight decline from more than $513,000 made in 2008.

Last year, Toyota sponsored the CBC Foundation conference. It also contributed $5,000 to the Discover the Real West Virginia Foundation on behalf of Sen. Jay Rockefeller (D-W.Va.), $2,000 for the William Clay Scholarship on behalf of Rep. William Lacy Clay (D-Mo.) and $541 for the Joe Baca Foundation on behalf of Rep. Joe Baca (D-Calif.).

Campaign finance experts say one reason for some of the decreases is attributable to recent changes in reporting regulations.

“The initial guidance was that if somebody simply bought tickets or tables to an event where a Senator was speaking, that it was an expense to honor the Senator,” said Jan Baran, a partner at Wiley Rein who specializes in campaign finance and lobbying ethics. “That is no longer the case, and it has not been the case for [months].”

Baran said other drops might be attributed to large one-time donations made in 2008 on behalf of President Barack Obama’s inauguration and other events honoring the new president. In 2008, lobbyists spent $3 million on behalf of Obama and Vice President Joseph Biden. The contributions on behalf of the president declined to $500,000 last year.

While the clarification of some regulations may have led to reporting of less money, another change has made disclosures increase. Lobbyists must now report the full cost of the event on their semiannual contribution reports and not just the nominal expense of the award or plaque bestowed on federal employees.

Such a change irked the U.S. Chamber of Commerce’s Institute for Legal Reform, which reported spending $146,861 for an event last October in which Senate Judiciary ranking member Jeff Sessions (R-Ala.) was honored.

Chamber officials complained they were required to report the full cost of an eight-hour conference even though Sessions spoke for only 30 minutes and received a crystal gavel worth $210. Steve Law, the general counsel for the chamber, said the $146,861 did not reflect the value of what the group’s Institute for Legal Reform did on behalf of the Senator.

“He gave a very brief 30-minute update. He barely got a chance to get a cup of coffee,” Law said, adding that while he supports full disclosure, “There is a point when too much information starts to become confusing and counterproductive.”