Nuclear power advocates are waging an intense lobbying campaign on Capitol Hill this week in an attempt to limit the political fallout from the reactor crisis in Japan, which threatens to undermine already shaky plans for expanded nuclear capacity in the United States.

Lobbyists with the Nuclear Energy Institute and some of the United States’ largest energy firms, including Exelon Corp. of Chicago, are holding meetings with key lawmakers and standing-room-only briefings for staffers in an attempt to tamp down talk of restrictions in response to the Japanese disaster.

The efforts come as lawmakers held hearings Wednesday focused on the impact of the worsening catastrophe at the Fukushima Daiichi power plant, where at least three reactor cores are believed to be imperiled following a major earthquake and tsunami last week.

The disaster has renewed calls from U.S. environmental groups and some lawmakers for a more cautious approach to nuclear power projects, which are central to the Obama administration’s plans for an energy strategy less dependent on high-polluting fossil fuels.

Nuclear industry lobbyists are telling lawmakers of both parties that the severity of the calamities facing the Japanese plant are virtually impossible to replicate in the United States. They also say that two plants located in quake-prone areas of California are built to higher standards than the decades-old Fukushima facility.

“To the extent we can learn lessons about what happened in Japan, we will import those lessons,” said Alex Flint, chief lobbyist for the nuclear institute, which has briefed about 50 members of Congress so far. “If changes need to be made, we will make those changes. The most important thing for us to get out as much information as we can.”

President Obama and some other Democrats have warmed to nuclear power in recent years because it does not produce high levels of greenhouse gases believed by most scientists to worsen global warming.

But environmental groups say the Fukushima crisis underscores the lethal dangers of relying on nuclear power plants, where a single disaster can imperil the health and lives of millions. Activists note that about a quarter of the United States’ 104 nuclear plants share the basic features of the endangered Japanese “boiling water” reactors, which were designed by General Electric.

“The American public has gotten a primer course, or for some of us a refresher course, on the hazards of radiation and nuclear power,” said Dave Hamilton, director of global warming and energy programs at the Sierra Club. “You only think nuclear is a good idea if you really don’t think about the risks.”

Rep. Ed Markey (D-Mass.) has called for a moratorium on additional reactors in quake-prone areas and new safety measures at existing plants. Markey has also asked Obama to enforce a law passed in 2002 to provide potassium iodide pills to residents within 20 miles of nuclear plants for use in case of radiation exposure.

Sen. Joseph Lieberman (I-Conn.), a nuclear power advocate, also grabbed headlines earlier this week when he appeared to call for a moratorium on new reactor construction; Lieberman backed down from that position Tuesday.

Most Republicans, meanwhile, remain enthusiastic boosters for nuclear power, even as they push to curtail spending at the Office of Nuclear Energy and elsewhere. The House budget bill passed earlier this year included more than $330 million in cuts for nuclear waste disposal, safety oversight and other programs, according to advocacy groups.

One bill, proposed by Rep. Devin Nunes (R-Calif.) and co-sponsored by more than 50 others, calls for 200 new nuclear power plants by 2040.

That measure is far more optimistic than industry estimates, which suggest that only about a half dozen new reactors are likely to be constructed by 2020. A package of nuclear plant subsidies and loan guarantees passed in 2005 has had little impact because of low natural gas prices and the massive costs associated with nuclear projects, experts said.

Thomas B. Cochran, senior nuclear scientist at the Natural Resources Defense Council, said the Fukushima disaster further dims the prospects for a “nuclear renaissance” in the United States.

“You can try to put lipstick on this pig, but it’s just not going to look good,” Cochran said. “You’re already seeing proponents on the Hill and in the Obama administration saying that we aren’t changing our course, but there’s just not going to be a lot of commitments in this environment.”

Spurred on by federal subsidies and potential climate-change legislation, the nuclear industry has dramatically stepped up its federal lobbying and campaign contributions in recent years, records show.

The Nuclear Energy Institute, for example, has spent more than $6 million on lobbying since 2008, employing more than 20 internal and external lobbyists including former Rep. Bob Walker (R-Pa.), according to the Center for Responsive Politics. The institute’s political-action committee also spent more than $470,000 during the 2010 cycle, primarily favoring Democrats then in control of Congress, the records show.

Although focused on the United States, the nuclear institute also has foreign members including Tokyo Electric Power Co., the firm that owns the Fukushima plant.

Flint said the institute has been working “24 hours a day nonstop” since the Fukushima crisis began to brief lawmakers and aides as well as state officials around the country.

“There’s been an evolution and a growth in support for nuclear energy in Congress, particularly among members trying to address climate-change issues,” Flint said. “There may be members who change their minds about nuclear energy, but for the time being all we’re focused on is getting information out.”

Another active player in this week’s lobbying push is Exelon, the Chicago-based utility that ranks as the largest owner of nuclear power plants in the nation, with 17 reactors in three states. Exelon, which helped run briefings on Monday for House and Senate staffers, spent $3.7 million on lobbying and $1.7 million through its PAC in 2010, with nearly 60 percent of contributions going to Democrats.

The company said in a statement this week that its plants “are equipped with numerous and redundant safety systems designed to protect them against earthquakes, flooding” and other disasters. “Our plants are operating safely, and our plant neighbors are safe,” said chief executive John Rowe.

Another major player in the nuclear industry is Duke Energy Corp., whose chief executive, Jim Rogers, is leading fundraising efforts for the Democratic National convention to be held in Charlotte, N.C. The firm has agreed to guarantee a $10 million line of credit for the convention from a local bank.