WASHINGTON — The Senate will embark on a free-wheeling immigration debate this week, testing the limits of whether there is still a political center on the contentious issue in the era of President Donald Trump, and whether a deal can be reached in a span of a few days.

The Senate voted Monday evening to advance that debate, the first step in potentially protecting hundreds of thousands of undocumented immigrants brought to the US illegally as children from deportation, but pressure from both the left and right could make it harder than ever to achieve something on immigration.

“This President ran on the most outspoken, anti-immigration platform of any presidential candidate in modern history. He said things during the course of the campaign, he’s done things since he was elected President ..and made it clear that this is the bedrock of his base support,” Sen. Dick Durbin, the Democratic whip, told reporters Monday evening. “We hadn’t faced something quite that organized and specific when we took this up five years ago. This has made it much more difficult.”

Unlike five years ago when the Senate debated comprehensive immigration reform on the floor of the Senate, the politics of the moment have been complicated by not only a President who made building a border wall and deporting immigrants the centerpiece of his campaign, but a Democratic base that is demanding more than ever that its members hold the line against the President.

Another complicating factor? Members and aides tell CNN that the debate is only expected to last a week, a tight deadline for a congressional body that relies on unanimous cooperation and is prone to dilatory tactics.

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“If you’re going to have an open debate, you have to let people have their amendments,” said Sen. Roy Blunt, a Republican from Missouri and member of leadership. “People can’t exercise their ability to object to people having amendments. The old-fashioned way to deal with an amendment is to vote on it.”

If members aren’t careful, members and aides warn time could be eaten up by arcane Senate rules and partisan grandstanding.

Still, as Arizona Republican Sen. Jeff Flake put it, “this is the best shot we’ve got.”

There are still many unanswered questions about how the debate will unfold. The number of amendments is still uncertain as are the contents of those amendments. Republican have introduced one amendment that closely resembles Trump’s immigration framework. And a bipartisan group of lawmakers is still trying to craft their own proposal to present for a vote. But Democrats haven’t unveiled their amendments at this point and frustration was growing Monday night from Republicans that Democrats still hadn’t laid down out their hand.

“Democrats have been talking about this for a long time,” said Sen. John Barrasso, a Republican from Wyoming and member of leadership. “You’d expect that they’d be ready.”

Majority Leader Mitch McConnell has been careful not to put his thumb on the scale of the debate. Instead, laying down a shell bill that had nothing to do with immigration that he has invited members to amend. The rules of the debate are simple: whoever can get 60 votes on their proposal, wins.

But lingering in members’ minds is the stark reality that it’s possible the week ends with no resolution at all on the expiring Deferred Action for Childhood Arrivals program. Weeks of bipartisan, bicameral meetings between the House and Senate vote counters haven’t yielded an agreement, the White House has shot down multiple bipartisan proposals and Durbin — a lead player in the negotiations — said Monday that he hasn’t been able to find 11 Republicans — a number of votes when combined with Democrats’ 49 can advance legislation — to support any of his efforts yet.

“I do not see a scenario in which any of these pass,” one Democratic Senate aide told CNN.

Even if they could come to a consensus there is no guarantee the House would agree nor is their any commitment the President would sign it.

The pressure is on both Republicans and Democrats to hold the line here. Trump has been clear that he wants any DACA proposal to also tackle border security, the diversity lottery and family-based migration. Anything short of that and Republicans run the risk of appearing softer on immigration than Trump and therefore prone to criticism from the base and conservative media of backing “amnesty.” Democrats, meanwhile, have been emphatic they won’t agree to lower legal immigration numbers as part of a DACA debate as they face mounting pressure from their base that is still irritated Democrats caved and voted to re-open the government after a brief shutdown in January over immigration.

If there’s a middle ground, it’s not clear yet. And in the era of Trump, it’s not even clear if there even is one one on immigration anymore.

Sen. Bob Menendez, a Democrat from New Jersey said the week was about trying to find a “nucleus of the Senate” and that would require Democrats to find enough moderate Republicans to pass a proposal.

“Either we will come to an understanding that there is a possibility for that or there isn’t,” Menendez said.

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