(Bloomberg) — Republicans who have vowed to roll back dozens of Obama-era coal, climate and energy regulations are up against a harsh reality: There isn’t enough time to kill them all.

A packed congressional calendar and the challenge of assembling a coalition to vote down each measure will force lawmakers to pick their targets. New cabinet secretaries can relax enforcement of certain regulations but can’t scrap them entirely without a lengthy rulemaking process.

And in some cases, such as a measure announced this week to restrict offshore oil drilling based on an obscure statute, Obama has employed novel means that may take years to untangle in court.

"More will happen than environmentalists want, but less will happen than Republicans are hoping," said Kevin Book, managing director of the Washington-based research firm ClearView Energy Partners.

Two Interior Department rules are the top candidates to get a vote under expedited procedures that give lawmakers a chance to review major regulations: one limiting the venting and flaring of natural gas on public lands and another designed to protect streams from coal-mining pollution. Both are opposed by industry.

The measures were part of a flurry of last-minute rulemaking by President Barack Obama, which has pushed nearly two dozen major regulations out the door since the election. With so many measures taking effect, not all may be repealed.

"Part of the Obama administration’s strategy is filling the policy horizon with chaff and hoping some of the missiles get through," Book said. "And it might work."

Republicans have made it a top priority to rescind the "job-killing" federal actions they say are throttling U.S. energy development, including a moratorium on new sales of federally owned coal and Obama’s Clean Power Plan, which slashes greenhouse gas emissions from power plants. On Wednesday Trump named billionaire investor Carl Icahn, a critic of EPA rules, to help him identify the "strangling regulations" that can be axed.

The regulations on flaring natural gas and stream protection are priorities for Republicans from western and coal-mining states in House and Senate leadership. On Monday, the day the Interior Department issued the stream rule, Senate Majority Leader Mitch McConnell, a Republican from coal-producing Kentucky, said taking it down would be a top priority next year.

The conservative House Freedom Caucus, the Senate Republican Policy Committee and other lawmaker groups have assembled wish lists identifying dozens of energy and environmental regulations to repeal. No target is too small or too large. The list includes the Clean Power Plan, as well as energy efficiency requirements for decorative gas fireplaces and battery chargers.

While lawmakers could advance traditional legislation to dismantle Obama’s environmental and energy portfolio, most bills would struggle to meet a 60-vote threshold necessary to cut off debate and advance major legislation in the Senate.

To get around that hurdle, congressional Republicans have at least two tools at their disposal. The first, called budget reconciliation, is a two-step process that involves adopting a budget resolution with specific policy instructions, so that later legislation is fast-tracked. But Republican leaders may have a hard time assembling a majority for a budget resolution containing a slew of controversial policy issues, and energy issues are going to have to slot in behind Obamacare and tax reform in importance.

Still, oil industry leaders are pressing Republicans to use budget reconciliation to reverse Obama’s decision Tuesday to withdraw more than 100 million acres in the U.S. Arctic and Atlantic oceans from future oil leasing based on authority from a 1953 law. Because that could be tricky for Trump to reverse, it may get a close look from Congress.

Lawmakers also can pass resolutions of disapproval under the 20-year-old Congressional Review Act, which provides a process to have filibuster-free votes on major rules. Regulations finalized as late as June 13 could be on the chopping block, according to the non-partisan Congressional Research Service, because the law applies to rules issued in the past 60 legislative days. While Obama has vetoed CRA measures during his presidency, Trump is sure to sign them.

In addition to the stream protection rule and methane venting regulation, top CRA targets include a measure to change the way royalties are calculated on coal, oil and natural gas from public lands and one that shortens the drilling seasons in the Arctic.

But even getting those voted on could be complicated. Under expedited procedures, each resolution of disapproval targeting an individual regulation for repeal will consume at least 10 hours of floor time in the Senate. That may not sound like much, but the Republican-led Congress will be crushed with other business, including confirming Trump’s cabinet nominees, vetting a Supreme Court candidate and finalizing a budget, not to mention tackling legislative priorities such as the Affordable Care Act, immigration, infrastructure and tax reform.

Also, Republicans hate so many of Obama’s energy rules, there may discord over which to tackle via the CRA.

"So many members are going to have competing priorities," said Robert Dillon, a former Senate Republican staff member now with the American Council for Capital Formation. "People are coming around to this idea that they need to be a little more strategic, a little more realistic."

Lobbyists already are gaming out what regulations are a better fit for Congress and which are best left for agencies to whittle down, even if it takes years.

"It takes longer for regulatory agencies to do things, but Congress’ time is precious," said Daniel Simmons, vice president for policy at the American Energy Alliance, and a member of the Trump transition team focused on the Energy Department.

Sam Batkins, a regulatory expert at the American Action Forum, a think tank that favors smaller government, says he would bet the Congressional Review Act is used successfully on just five to six rules, including some outside of energy policy. The measure has only been used successfully once before: to repeal Clinton-era ergonomics rules when President George W. Bush took office.

For most rules, companies and industries looking to Washington for regulatory relief might have better odds appealing directly to the executive agencies that administer them.

"If you’re waiting on Congress, it’s a waiting game, and they’re probably not going to get to you," said Bloomberg Intelligence analyst Rob Barnett.

Legal challenges have already been mounted against a host of Obama administration environmental rules, including the Clean Power Plan and methane mandates. The Trump administration could stop defending those measures in court or not appeal a ruling against them if it comes.

Trump and his executive agencies will have wide latitude to write new rules — just not immediately. Dismantling mandates, just like enshrining them, requires drafting new regulations and subjecting them to public scrutiny, a process that can take years. And any dramatic 180-degree reversals may only survive later legal scrutiny if there is solid evidence supporting the changes.

John Walke, a senior attorney with the Natural Resource Defense Council, said even if Congress does kill a regulation, the law may require the EPA to regulate in that area.

"If an agency were disinclined to restart a rule or took the resolution of disapproval as permission not to restart a rule, then health and environmental groups would sue the agency to make sure the statute is upheld and enforced," he said.

Bloomberg News by Jennifer A. Dlouhy and Ari Natter