Washington (CNN) The high stakes battle over immigration policy has politicians and thinkers on all sides of the spectrum preparing for battles that could last years -- leaving millions of people in the US unsure of whether they'll be able to stay in the country.

The President-elect has made immigration a focal point throughout his campaign, from his first announcement that he was running in the 2016 race -- where he immediately sparked controversy with accusations that some Mexican immigrants were criminals and rapists.

From pledging to build a wall along the Mexican border to saying he would mass-deport millions of immigrants living in the US illegally and saying he would block foreign Muslims from entering the country (a position he later moderated), Trump has repeatedly pledged a hard-line stance on immigration as one of the key reasons to vote for him.

The hard-line statements with shifting details have left millions of Americans uncertain about their future status in the US. More than 700,000 undocumented immigrants who came to the US as children are protected currently by a deferred action program established by President Barack Obama, and another 4 million to 5 million were eligible for protection under a similar program for parents of US citizens and lawful residents that was blocked by federal courts. Many of the families' information could be in federal systems, allowing for targeted removal under a Trump administration.

Since being elected, Trump has signaled he may take a softer approach on some issues rather than the strident tone he struck during the campaign.

"We're going to work something out that's going to make people happy and proud," Trump told Time magazine about the so-called DREAMers, young people brought to the US as children who meet certain education and work requirements. "They got brought here at a very young age, they've worked here, they've gone to school here. Some were good students. Some have wonderful jobs. And they're in never-never land because they don't know what's going to happen."

Here's a look at where some of the fights on immigration policy may take place under President Trump.

Executive actions

There are a number of actions Trump can take his first day in office. He has pledged to roll back Obama's immigration executive actions, which would include deferred action, on Day One. He also has pledged to step up enforcement of deportation orders and immigration laws, which he could immediately direct federal enforcement agencies to do.

Trump also would have authority to instruct agencies to up their screening for visa applicants, including refugees, said Stephen Yale-Loehr, an immigration attorney and Cornell Law professor. While Trump likely couldn't block entire groups from non-refugee visas without congressional action, he could unilaterally mandate high-level scrutiny on applicants from certain countries, making the process difficult for them to enter the US.

He could also immediately kick off the rulemaking process to roll back Obama-era regulations, like one making it easier for students to extend their authorization to study in the US if they go into a science, technology or math field.

Once Trump's Cabinet officials are confirmed by the Senate, they will also have latitude within their agencies, especially in the departments of Justice and Homeland Security, which work together to enforce immigration laws. For example, the attorney general can overrule decisions by the Board of Immigration Appeals, the highest immigration court in the land, and has the authority to appoint judges to it, as Washington University professor Stephen Legomsky points out. Legomsky has consulted with Democrats on immigration.

Trump has selected Alabama Republican Sen. Jeff Sessions, an immigration hard-liner, as his nominee for attorney general, and retired Gen. John Kelly, whose immigration policy positions are little known, for Homeland Security.

"Given his nominees for both the Justice Department and DHS, I think they're going to full bore ahead with day one telling the border patrol: 'Start enforcing the law,'" said Hans von Spakovsky a legal and immigration expert at the conservative Heritage Foundation.

Von Spakovsky said the enforcement could entail turning people away at the border, locking up and deporting anyone who is caught in the US illegally and moving to deport those that the government already knows are here illegally, including people who may have registered for deferred action.

"All of that can be done on day one and that's going to immediately start not only reducing the population that's in the country, but it will have an immediate effect on the flow into the country because the message will go out."

Legislative fights

Beyond regulations and executive actions, many of Trump's policies will require acts of Congress.

Building a wall along the border, or simply extending the fence that already exists or beefing up security, will likely require appropriations from Congress to pay for it. And restricting the number of visas given to people entering the country legally would also require legislation.

Already, some legislative fights are emerging. Sens. Lindsey Graham, a Republican from South Carolina, and Dick Durbin, a Democrat from Illinois, have introduced legislation that would extend the protections of deferred action to immigrants brought to the US as children. It's unclear if that would get a vote in the Republican-controlled Congress next year.

In 2013, the Senate passed a bipartisan immigration reform bill negotiated by eight senators, all of whom are still serving in Congress. But the House never acted on the bill, arguing for a piece-by-piece approach that would start with border security.

House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell will likely take their cues from Trump, but the first steps could be putting money in a budget bill that will appropriate government funding starting in April to beef up enforcement and border security.

But if lawmakers try to start putting together immigration legislation, they will likely end up needing a comprehensive package to address how it's paid for and how it works together, said Stephanie Martz, a lobbyist who worked as a key staffer on the Gang of Eight bill when she worked for New York Democratic Sen. Chuck Schumer -- now the minority leader.

"You start pulling on the thread of the sweater and you realize quickly you can't do it piece by piece," Martz said. "These issues are always like a giant Rubik's cube, you often hear Republicans (say what) needs to happen is for things to be passed piecemeal -- 'If we secure the border first, we can do other things' -- but you need to figure out how to pay for border security." The Gang of Eight bill raised money be charging more for visas and reforming the legal immigration system.

Conservative Republicans largely opposed the bill because it created a way for people living in the US illegally to seek citizenship.

Another piece of legislation that could move quickly could be what's known as "Kate's Law," named after Kate Steinle, a young woman who was shot and killed by an undocumented immigrant that had repeatedly violated his deportation order and re-entered the country. The bill would institute escalating mandatory minimum prison sentences for anyone caught illegally re-entering the country in violation of a deportation order.

Von Spakovsky predicted that further legislation and more aggressive deportation may wait until the results of executive-led stepped-up enforcement become clear. Incoming White House chief of staff Reince Priebus did not list immigration in a recent interview with Hugh Hewitt where he laid out the legislative priorities of the first few months of next year.

Sanctuary cities

Beyond the Beltway, major fights are already shaping between state and local governments and the federal government.

Many cities have designated themselves as "sanctuaries," meaning they will not help the federal government enforce immigration laws. Efforts include not expending law enforcement resources on catching undocumented immigrants, and refusing to turn over information about undocumented individuals to the feds.

The city of Chicago approved a $1.3 million budget amendment last week to create a legal protection fund for immigrants, and the mayors of New York and Los Angeles signed a letter with more than a dozen others that Chicago Mayor Rahm Emanuel presented to the President-elect at Trump Tower asking him to extend the deferred action program.

Conservatives have threatened to punish sanctuary cities and states by cutting off federal funds if they don't cooperate. That could include DOJ and DHS grants, and Congress could vote to remove more federal funds. Von Spakovsky said the government could even sue sanctuary cities to turn over the names of undocumented residents.

"Any city mayor who defies that court order, they could be held in contempt, they can be fined or jailed, the same way defiant mayors in the civil rights era were fined and jailed for defying court orders," von Spakovsky said.

Court battles

Sanctuary policies, which are also being pursued on college campuses, are almost sure to spur court action, both from the feds trying to enforce policies and local governments fighting back if funds are taken away.

But it won't just be sanctuary fights that tie up the courts.

Just as Texas successfully blocked Obama's effort to extend deferred action to parents through protracted legal battles, Democrats could take a page from Republicans' book and use litigation to slow down immigration rules and laws.

The attorneys general of blue states such as Illinois, New York and California are expected to be on the front lines of challenging Trump's immigration policies, much as red states challenged those of Obama. In one sign of the prominence these battles could take, California Rep. Xavier Becerra opted to leave Congress, where he had a term-limited leadership position, to serve as California's attorney general.

Martz said there is plenty of ammunition for civil rights and pro-immigration groups to fight any legislation or executive action that is perceived to violate bedrock American principles, as well.

"I definitely think we're going to see litigation," Martz said. "There's plenty of law that supports that, there are anti-discrimination laws, there are constitutional provisions that prevent doing anything entirely race based, so we'll see."

And Yale-Loehr said some actions, like college campuses all making declarations of sanctuary for their students, even if they aren't strong legally, are designed to win the court of public opinion.

"There's legislation, litigation and public opinion as there is on any issue, and I think all three avenues will be deployed," Yale-Loehr said.