President Barack Obama announced a sweeping plan to curb auto emissions on Tuesday, once again sending Congress a message that he’s not waiting for it to act on his major priorities.

Obama’s expansive view of executive power is starting to worry moderate Democrats and Republicans who fear he will impose by regulation what he can’t get via legislation.

Obama has already taken a host of actions without explicit Congressional approval — from guaranteeing the warranties of struggling automakers to announcing his plans to close the Guantánamo Bay, Cuba, prison.

But with climate change legislation uncertain to pass Congress, Obama could use the Environmental Protection Agency’s ability to regulate greenhouse gas emissions to accomplish a similar result.

The administration has repeatedly said it prefers a bill, but it has left the regulatory option on the table — one it started wielding on Tuesday.

That’s one reason industry groups and moderate Democrats have been trying to cut business-friendly deals with Energy and Commerce Chairman Henry Waxman (D-Calif.) and pass a climate change bill they can live with, rather than face the uncertainty of an EPA regulation.

For some moderate Democrats like Sen. Ben Nelson (Neb.), the precedent set Tuesday is troubling.

“I am very concerned about any expansion of the authority of the EPA to impose standards where Congress is currently divided,— he said. “They’re not the fourth branch of government.—

Nelson, who is skeptical of “cap-and-trade— legislation but wants to see an energy bill that promotes domestic energy sources, said the administration should not act without Congress on board.

“If we are unable to do that, I don’t think that justifies the executive to go ahead and do it,— he said.

Sen. Debbie Stabenow (D-Mich.) praised the deal the president reached on mileage standards but warned Obama not to go further without coming to Congress first.

“It would be a serious mistake for the EPA to move independently,— she said.

But other top Senate Democrats, including Majority Whip Dick Durbin (Ill.) and Environment and Public Works Chairman Barbara Boxer (Calif.), said Obama can justify regulating carbon emissions if Congress fails to act, noting the Supreme Court’s ruling two years ago requiring the EPA to weigh in on greenhouse gases.

“What President Obama is doing is following the law,— Boxer said. “He has to act.—

Durbin said Congress has a responsibility to act, but history shows a failure to do so “cedes the battlefield to the executive and the judiciary.—

The reality for Democrats is that Obama’s aggressiveness has advantages, too — it helps them get votes from moderates and encourages industries to come to the negotiating table.

“Having a president who supports quick and aggressive action is only going to help us pass legislation,— said a House Democratic aide.

And in cases like the auto industry takeover, many Democrats quietly prefer to avoid politically unpalatable votes and are happy to have Obama take the heat.

But Republican lawmakers charge that Democrats who criticized President George W. Bush for acting without Congressional approval or oversight are now cheering on Obama.

“There is a double standard here, now that the shoe is on the other foot,— Sen. John Thune (R-S.D.) said. He said the administration has acted as if it “can just wave a magic wand,— and he complained about a range of Obama’s decisions, from executive pay regulations to new biofuels rules.

Thune said it’s obvious the threat of regulatory action is being used to whip up support for the cap-and-trade bill. “They’re kind of playing this good cop, bad cop,— he said. “They are using every bit of running room that they have.—

Sen. James Inhofe (R-Okla.), the ranking member on the Environment and Public Works panel and perhaps the Senate’s chief global warming skeptic, said Obama is “going to go industry by industry— with new regulations.

“They have been doing this since they have control over everything,— he said. “That’s why they have so many czars who don’t need Senate confirmation.—