A long-anticipated showdown on immigration reform is coming this week — and nobody knows how it will turn out.

The Senate is set to begin debate Monday night on an issue that has vexed lawmakers for years, probably signaling whether the closely divided chamber has any hope of striking a bipartisan compromise.

Among other challenges is whether Congress can find a way to protect “dreamers” — as a majority of Americans want for those young undocumented immigrants brought to the United States as children — while also enacting changes in border security eagerly sought by President Trump.

“We’re going to have something in the Senate that we haven’t had in a while,” Sen. Jeff Flake (R-Ariz.) said Sunday on NBC’s “Meet the Press.” “It’s a real debate on an issue where we really don’t know what the outcome is going to be.”

And few are saying much publicly about what they’re planning.

“There’s not a lot of deep planning that’s gone on,” said Frank Sharry, founder and executive director of America’s Voice, an immigration advocacy organization. “Everyone was focused on what was going on with the shutdown. I think it is going to have a helter-skelter quality to it.”

Even if the Senate is able to pass a bill, it’s far from certain that the House will move ahead with it. Speaker Paul D. Ryan (R-Wis.) said last week that the House “will bring a solution to the floor, one the president will sign.”

What exactly Trump will support remains crucial yet unknown, as he has shown little willingness to accept anything short of the four-part plan he proposed last month.

In a weekend tweet, he reiterated support for “creating a safe, modern and lawful immigration system” that includes more border security, ending family-based legal migration and ending the diversity lottery program. He made no mention of his support for protecting 1.8 million dreamers, whose status was thrown into uncertainty when he canceled an Obama-era program known as Deferred Action for Childhood Arrivals, or DACA.

“It’s time for Congress to act and to protect Americans,” Trump said in a video message released late Saturday. “Every member of Congress should choose the side of law enforcement and the side of the American people. That’s the way it has to be.”

Trump sparked the debate in September by announcing the end of DACA, which grants temporary legal status to about 690,000 dreamers. He has given lawmakers until March 5 to enact a permanent solution.

But Congress has failed for years to secure the votes to pass a Dream Act, as the legislation has become known.

Supporters of such legislation had hoped to tie it to the debate over spending, which has prompted two brief government shutdowns in recent weeks. Although that didn’t happen, Senate Majority Leader Mitch McConnell (R-Ky.) did agree to set the immigration debate in motion last month when, facing pressure from senators in both parties, he said he would permit votes on immigration proposals in exchange for ending the first shutdown, last month, which lasted three days.

“I’m not trying to tilt the playing field in any particular direction,” he said last week when asked about the debate.

Unlike most congressional debates, which begin with a prepared piece of legislation, the give-and-take over immigration will not. Instead, McConnell used his powers as floor leader late last week to bring up an unrelated bill that he said will be used as the “shell” for the debate. The shell can be reshaped when a proposed amendment has the 60 votes needed to clear procedural challenges and pass. Once amendments are added, the final bill will also require at least 60 votes to survive and pass.

“Every ounce of energy this week is going to be spent on crafting a bill that protects dreamers and can get 60 votes,” Senate Minority Leader Charles E. Schumer (D-N.Y.) said in a statement. “It’s a hard needle to thread, but we are making progress.”

Flake said in a separate interview last week that immigration “is something that Mitch has been loath to address; we know that.” The forthcoming debate “may not yield anything — that’s dangerous in and of itself — but we just don’t know what coalitions will develop and what amendments will gain steam.”

Liberal organizations and immigration reform advocates are warily watching the debate, pushing for a narrow fix to protect dreamers and warning that they will hold Democrats and vulnerable Republicans accountable if they cannot keep Trump’s proposed policy changes to a minimum.

“Our nightmare scenario is that we get into a long-term conversation about immigration,” said Angel Padilla, policy director for Indivisible, a grass-roots liberal organization. “There are things that need to be addressed for sure that should be addressed separately, but that will only block actual real solutions for dreamers.”

Aides in both parties and advocates tracking the debate expect that Democrats and Republicans will try introducing proposals to test the Senate’s appetite for reform.

Late Sunday, seven GOP senators — Charles E. Grassley (Iowa), John Cornyn (Tex.), Thom Tillis (N.C.), David Perdue (Ga.), James Lankford (Okla.), Tom Cotton (Ark.) and Joni Ernst (Iowa) — introduced the Secure and Succeed Act, a plan that mirrors Trump’s proposals.

The bill would legalize 1.8 million dreamers and authorize $25 billion for “physical and virtual” fencing and other technology along the southern border and funding for more Border Patrol agents. The legislation also would limit family-based legal migration to just the nuclear family and reallocate visas from the diversity lottery program to other visa programs.

Grassley, who chairs the Senate Judiciary Committee, called the plan “a reasonable approach” and “a rare opportunity to fix a real problem and protect the country in a thoughtful and compassionate way.”

Meanwhile, Sen. Susan Collins (R-Maine) has been hosting meetings in her office since last month’s shutdown, trying to get about 25 senators in a bipartisan “Common Sense Caucus” to endorse a plan that could pass overwhelmingly. After several long meetings fueled by several boxes of Girl Scout cookies, they still have nothing.

“I don’t know whether we can get there or not,” she said.

Sen. Amy Klobuchar (D-Minn.), a member of the group, said the talks are “a true test for the U.S. Senate: Will senators go to the opposite corners of the partisan boxing ring, or will they come together to resolve a critical issue?”

A bipartisan proposal by Sens. John McCain (R-Ariz.) and Christopher A. Coons (D-Del.) has been dismissed by Trump as a “waste of time.” It would grant legal status to a larger pool of undocumented immigrants than the 1.8 million Trump supports legalizing and not immediately authorize spending the $25 billion Trump wants to fortify the U.S.-Mexico border. Their bill also says nothing about curbing family-based legal migration or making changes to the diversity lottery program.

Democrats, meanwhile, are expected to introduce a new version of the Dream Act, a bill first introduced during George W. Bush’s presidency that would provide a pathway to citizenship for millions of dreamers. A majority of Americans support the concept, but it is opposed by most Republicans unless it is passed alongside changes in border security.

Sen. Richard J. Durbin of Illinois, the chief Democratic negotiator on immigration, is the longtime lead sponsor of the Dream Act. If he doesn’t introduce it, Democratic Sens. Kamala D. Harris (Calif.), Cory Booker (N.J.) or Kirsten Gillibrand (N.Y.), or Sen. Bernie Sanders (I-Vt.) — all of whom are mulling 2020 presidential bids — might assume the mantle.

Divisions among Republican senators have flared in recent weeks over what should be in an immigration bill, adding to the complications of securing an agreement.

Sen. John Thune (R-S.D.) said at a recent GOP congressional retreat that a narrow measure addressing the fate of young immigrants and border security may be the best deal the Senate can pass. But Lankford has said he has no interest in what he termed a “skinny” immigration framework.

“Everybody's trying to figure out the chaos of next week. So I don't know yet how open the process is going to be,” Lankford said.

Seen as a bridge between conservatives in his party and the bipartisan group, Lankford is also pushing an idea to grant dreamers five years of “conditional permanent residency” so long as they remain employed, in school or serving in the military. If they maintained that status for 10 years, they could then seek a green card. And five years after that, they could apply for citizenship.

Perdue and Cotton, two of Trump’s staunchest allies, have also been seeking to curb legal immigration — an idea long championed by White House domestic policy aide Stephen Miller but opposed by other GOP lawmakers.

One wild card is Sen. Marco Rubio (R-Fla.), an architect of the comprehensive immigration deal that passed the Senate in 2013 but died in the House. Rubio, who struggled to explain his effort to the conservative base when he ran for president in 2016, has taken more of a back seat in the talks this time around.

The immigration debate “is like a Rubik’s Cube,” Rubio said. “I mean, every time you line up the red side of the Rubik’s Cube, the blue side is off balance, and vice versa.”

Mike DeBonis, Sean Sullivan and David Weigel contributed to this report.