Some allies of Sen. Bernie Sanders are beginning to map out how he might govern and what his administration could look like if he wins the presidential election — a once inconceivable concept for many congressional Republicans and Democrats who regard his ideas as impractical.

Independent advisers in contact with Sanders’s campaign are informally speculating about who could be tapped to lead key agencies such as the Treasury Department, what legislation would be prioritized and which executive orders Sanders would approve amid congressional resistance to his agenda.

Sanders (I-Vt.), who won the New Hampshire Democratic primary Tuesday night after a top showing in Iowa last week, has proposed more than $50 trillion in federal spending and vast new government mandates — including a national rent-control standard and a ban on exporting crude oil — that have until recently been far outside the Democratic mainstream.

Interviews with more than a dozen internal and external advisers reveal how his campaign team is squarely focused on locking up the Democratic primary, but his success has led others to try to gauge what might be next. Many of them spoke on the condition of anonymity because the process remains so fluid and many decisions have not been made.

His inner circle of advisers, which includes aides to former Senate majority leader Harry M. Reid (D-Nev.) and a number of Washington outsiders, have not started designing a potential administration. They cautioned that such talk is premature and not occurring in the campaign, which they stressed is focused on winning the nomination and defeating President Trump.

But Sanders’s momentum during the primary campaign has led allies to debate ways he could put his ideas into action. And as he rises in national polling, Sanders is expected to face greater scrutiny about how he could translate his sprawling domestic policy agenda into legislative reality. His 2020 campaign has relied heavily on outside experts and advocacy groups once confined to the fringes of Democratic politics, while also elevating to key positions those with closer ties to the party establishment.

Some parts of Sanders’s policy agenda remain unchanged from 2016 — advisers say Medicare-for-all and a $15-an-hour minimum wage would probably top his list of first priorities. He has expanded his 2016 agenda by adding policies such as a Green New Deal, a housing guarantee for all Americans, the elimination of all student debt held in the country, and an aggressive wealth tax on multimillionaires and billionaires.

Critics say it is unclear whether Sanders’s team is up to the daunting legislative and policy challenges ahead, while supporters see a major asset in their rejection of the traditional Democratic channels and policy priorities.

Sanders has hired three former aides to Reid, who had a knack for holding the line during public debates while also cutting deals with Republicans at opportune times. Faiz Shakir, Sanders’s campaign manager, is seen as bringing a steadying approach to the senior leadership of the campaign, while deputy campaign manager Ari Rabin-Havt and national policy director Josh Orton were also brought in from “Team Reid.”

At the same time, Sanders has retained longtime advisers in key roles. Warren Gunnels, who has served as an aide to Sanders for more than two decades, is viewed as one of the most crucial voices in stewarding his economic policy agenda, in part because of the loyalty Gunnels showed Sanders even when he was a little-known House backbencher from Burlington, Vt. Jane Sanders, the senator’s wife, has also helped craft the 2020 campaign’s climate and housing policies.

Bernie Sanders’s advisers have clashed repeatedly with the major Democratic think tanks that have traditionally driven Democratic policymaking.

His policy team, working with national political director Analilia Mejia, has instead consulted with environmental and immigration activist organizations such as the Sunrise Movement and the Center for Popular Democracy. It has also worked with labor-funded groups such as the Economic Policy Institute and the Democracy Collaborative, an international organization connected to leftist Jeremy Corbyn’s Labour Party in Britain, and new think tanks such as the People’s Policy Project, which was founded in 2017 and is funded by small-dollar donations. These groups, regarded as marginal players among Washington insiders, could gain new prominence should Sanders run the table in the election.

Sanders’s staff has also increasingly turned to a clutch of economic advisers who have largely flown below the radar, such as Darrick Hamilton of Ohio State University on economic and jobs policy, Tara Raghuveer of the People’s Action network on housing policy, Sarah Anderson of the Institute for Policy Studies on business taxes and Carol Zabin of the University of California at Berkeley on Sanders’s Green New Deal proposal.

Other economists who have previously had a hand in the Sanders campaign are also a source of speculation, by people who work with the campaign, about roles in the White House or the Treasury Department should he win the election. Some of the most frequently cited names for top economic posts include Jeffrey Sachs of Columbia University; Stephanie Kelton, a former Sanders aide who is now at Stony Brook University; Robert Reich, a Clinton administration labor secretary; and Joseph Stiglitz, a Nobel laureate also at Columbia.

The economists and experts in Sanders’s orbit in general have less White House and Washington experience than those at large liberal think tanks such as the Center for American Progress and the Center on Budget and Policy Priorities, some of whose leaders have historically gone on to fill crucial posts for previous Democratic presidents.

Skeptics say Sanders’s rejection of the typical Democratic policies and institutions may make it difficult to enact legislation, arguing that he has not passed much legislation through Congress during his approximately three decades there. (Sanders has pointed to his record of passing bipartisan amendments through a GOP-led Congress, as well as his recent work with Republicans on the war in Yemen.) Trump faced similar tensions in his takeover of the White House and the Republican Party, with bitter battles erupting routinely between long-standing GOP aides and Washington outsiders who disagreed about his direction.

“You’re clearly going to get people with less Washington experience in a Sanders administration, which means they’ll bring a fresh eye to new ideas and problems,” said Howard Gleckman, a policy expert at the Tax Policy Center, a nonpartisan think tank. But they “may also know less about how to get things done,” he said.

Campaign officials defended circumventing the traditional Democratic think tanks, arguing that they are conflicted by corporate donations and millionaire donors from which Sanders is trying to break the party free.

“In this campaign, Bernie and our staff are building policy in an unprecedented, grass roots way: by listening directly to the needs of people and generating plans that will actually solve the problems caused by toxic greed of powerful interests,” Shakir, the campaign manager, said in a statement. “This campaign is not taking its cues from the same Washington apparatus that has developed policy that for too long has left millions of working people, young people, and people of color behind.”

The scale of policies envisioned would be extremely difficult to execute, experts said. Democratic Party leaders remain dubious of Sanders’s political viability and, in some cases, the breadth and scale of his agenda. Republicans have railed against Sanders’s policies as representing a massive government takeover that would undermine American capitalism, with Trump warning against socialism in his State of the Union address.

Asked about Sanders on Tuesday, Senate Minority Leader Charles E. Schumer (D-N.Y.) and House Speaker Nancy Pelosi (D-Calif.) declined to criticize him. Sen. Richard J. Durbin (Ill.), the second-ranking Democrat in the Senate, played down Sanders’s showing in Iowa and said “some aspects of his proposals I just don’t think are practical.”

Timothy Naftali, a historian at New York University, said: “There is a major difference between being the leader of a movement and being the head of government. Could Senator Sanders build a coalition in Congress — that does not exist at the moment — big enough to pass his legislation? Right now, Sanders’s ‘democratic socialism’ appears to define only a minority within the Democratic Party.”

Some Sanders allies acknowledge the difficulty they would face in persuading Democratic lawmakers and institutions to help push their agenda. A Sanders administration would also be likely to precipitate immediate battles with some of the most powerful institutions in American life — such as the U.S. military, given Sanders’s pledge to shrink the defense budget, and the government’s immigration enforcement apparatus, whose power Sanders has vowed he would curb.

Although he has dozens of legislative proposals, Sanders has promised that he would send Medicare-for-all legislation to Congress in his first week in office — even though many if not most congressional Democrats oppose the effort to nationalize the country’s health insurance industry. Sanders’s $30 trillion single-payer bill would force every American onto a government insurance plan within four years, while expanding the Medicare program to provide dental, vision and other forms of care at no cost.

A multitrillion-dollar infrastructure package aimed at addressing climate change would also probably be one of the first measures pushed in a Sanders White House, as well as policies aimed at establishing a $15-an-hour minimum wage and increasing the percentage of American workers who belong to a union. Multitrillion-dollar housing and education programs, as well as a criminal-justice overhaul, are also expected to top the list.

These bills could face steep odds unless Sanders could find many more allies in Congress. Only a small minority Democratic senators agree with the need to eliminate private health insurance to establish a single-payer system, while just 14 Senate Democrats have co-sponsored the Green New Deal legislation. Sanders could lose even more support if he chose key congressional allies — such as Rep. Pramila Jayapal (D-Wash.), his national health policy co-chair — to fill spots in his administration.

There are other strategies Sanders might adopt if he were to win in November. He has said he would leave Washington routinely to join striking workers on the picket line in a way that would be unusual if not unprecedented for a president. Top campaign aides have prepared a list of possible executive orders for Sanders to implement if elected, including ending construction on Trump’s border wall and reinstating legal status for immigrants brought to the United States illegally as children, aiming to bypass Congress altogether.

Sanders’s list of potential unilateral actions, recently obtained by The Washington Post, includes criminal-justice measures such as legalizing marijuana in all 50 states.

Although many Democrats and Republicans have for years considered a Sanders presidency extremely improbable, his success in Iowa and New Hampshire may give them pause. Some allies also point to how quickly Trump reshaped the Republican Party on issues such as trade and immigration — and wonder whether a similar realignment would be possible in the Democratic Party under Sanders.

“If you were sitting there in 1931 or 1963, you could not have predicted what FDR’s New Deal or LBJ’s Great Society would be,” said Rep. Ro Khanna (D-Calif.), Sanders’s campaign co-chair.

Erica Werner contributed to this report.