President Obama is putting the finishing touches on his regulatory legacy.

In recent weeks, the administration has issued a flurry of new rules to expand overtime pay to millions of Americans, combat climate change with stronger standards for methane emissions and protect senior investors with new requirements for financial advisers.

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While the Obama administration is expected to pursue new rules aggressively in the months ahead, the window for issuing major regulations is nearly closed.

From the initial drafting phase to final approval, major regulations can take months, and in some cases years, to complete. And from this point forward, the next Congress would be able to overturn major rules passed under Obama, particularly if presumptive Republican presidential nominee Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE wins the White House.

Even if a Democrat wins, experts say there’s no guarantee they’ll have the same priorities.

“There are some rules that are not controversial that risk not being picked up by the next administration,” said James Goodwin, a senior policy analyst with the Center for Progressive Reform. He said a number of rules haven’t gotten the attention they deserve, including the Environmental Protection Agency’s regulation requiring chemical plants to have risk management plans and new rules for storm water discharge.

After Democrats lost the Senate in the 2014 midterm elections, Obama vowed that he would use executive power aggressively, if needed, to enact his agenda despite Republican control of Congress.

“I’ve got a pen and I’ve got a phone, and I can use that pen to sign executive orders and take executive actions and administrative actions that move the ball forward,” Obama said in January 2014.

“So one of the things that I’m going to be talking to my Cabinet about is how do we use all the tools available to us, not just legislation, in order to advance a mission that I think unifies all Americans,” he said.

Since January 2014, the administration finalized a rule that forces employers to disclose outside consultants they hire to counter workers union organizing efforts, raised the minimum wage for federally contracted employees, issued first-ever regulations for cigars and electronic cigarettes, and completed an overhaul of the country’s food safety system.

Supporters of Obama have praised the “pen and phone” approach, calling it a necessary response to obstruction in Congress.

“Congress has in fact been hostile and uncooperative and focused on rolling back vital public protections rather than taking on the nation’s urgent challenges,” said Robert Weissman, president at Public Citizen.

“We applaud the administration for its efforts to avert catastrophic climate change, prevent consumers from being ripped off, ensure employees are properly compensated, enhance economic stability and more. Fully addressing these underlying issues requires Congressional action; but the country is much better off for the administration doing what it could through regulation.”

But Republicans have blasted Obama’s use of executive power, saying he has overstepped his authority on issue after issue, most notably with an immigration order that is now before the Supreme Court.

“President Obama’s decision to ignore the limits placed on his power and act unilaterally to rewrite our nation’s immigration laws are an affront to the Constitution,” House Judiciary Committee Chairman Bob Goodlatte Robert (Bob) William GoodlatteNo documents? Hoping for legalization? Be wary of Joe Biden Press: Trump's final presidential pardon: himself USCIS chief Cuccinelli blames Paul Ryan for immigration inaction MORE (R-Va.) said in a statement last month.

“Such lawlessness must be stopped so that we preserve the separation of powers in the Constitution and protect individual liberty.”

Legal experts believe there’s a strong chance that the Supreme Court will deadlock on Obama’s move to shield some illegal immigrants from deportation, effectively leaving it in limbo until a new president takes office.

And immigration isn’t the only area where the Obama administration’s regulatory actions could be rolled back.

Some of the administration’s most controversial rules, including those to curb water pollution, have been challenged in court, and opponents say more legal challenges are to come.

Just last week, a coalition of nine financial groups filed a lawsuit challenging the Labor Department’s so-called fiduciary rule, and a financial services trade group threatened to sue the Consumer Financial Protection Bureau over its proposed rules for payday lenders.

Business groups, like many in the GOP, have bristled at what they say is overreach from the administration since Obama won reelection.

The question of whether Obama has used regulatory power more aggressively than his predecessors is fiercely debated in Washington.

James Gattuso, a senior research fellow in regulatory policy at the Heritage Foundation, said Obama is either at the top or near it in terms of the number of regulations issued compared to past administrations.

“Certainly it’s been one of the most active administrations in regulatory policy,” he said. “If you put together the effects of ObamaCare, Dodd-Frank and his climate change agenda, you have a massive increase of regulations.”

Regulatory proponents counter that Obama’s rulemaking is on par with past presidents like George W. Bush. They say the number of regulations under Obama is low, given the two huge laws that were passed during his tenure.

“It’s notable there’s not more regulations given Dodd-Frank and the Affordable Care Act, and the fact that the only way for the president to influence policy post the 2010 election was through rulemaking,” Weissman said.

“It’s remarkable how little regulatory action there has been against the backdrop of those two major statutes and the inability to move legislation.”

The White House just last month released a unified agenda that will serve as a roadmap for the regulatory agenda for the second half of 2016. It contained several notable regulations, including new requirements for renewable fuels, new fuel standards for medium- and heavy-duty vehicles, new energy-efficiency standards for ceiling fans, dishwashers and central air conditioners, and a rule extending paid sick leave to federal contractors.

Sam Batkins, director of regulatory policy at the conservative American Action Forum, estimates that another $113 billion in costs will come from the regulations Obama plans to issue in his final months. He said there are 14 new economically significant rules in the agenda, 10 of which are due to be issued in August.

Economically significant rules are those that carry an economic impact of $100 million or more.

What remains to be seen is whether the administration will dabble in “midnight regulations,” when agencies rush rules out the door before a new president takes office.

The White House’s Office of Information and Regulatory Affairs has already warned agencies to refrain from midnight regulations, releasing a memo back in January advising the federal agencies to finish their highest priority rulemakings this summer.

Experts at George Washington University’s Regulatory Studies Center predict the end of Obama’s term will be busier than usual when it comes to regulations.

In a report due out later this month, Sofie Miller, a senior policy analyst, said she found a 90 percent chance that Obama will issue more than the 72 economically significant rules that are released, on average, at the end of a presidency.

“He’s had an ambitious rulemaking agenda,” Miller said.

Others say the term midnight regulation is unfairly used to tarnish legitimate regulatory work.

“Midnight regulations are genuinely last-minute things done by jumping through or circumventing the normal and appropriate rulemaking process,” Weissman said. “The administration has been clear it’s not going to issue rules that circumvent the appropriate process.”