(AP Photo/Pablo Martinez Monsivais)

Nobody knows yet — probably including President Trump himself — how far his administration’s plans for mass deportations will actually go. The blueprint for stepped-up deportations that his Department of Homeland Security released this week vastly expands the pool of undocumented immigrants who can be targeted for removal, but what actually happens will depend on a host of unpredictable factors.

One of those centers on money. And it gives Democrats a way to try to resist Trump’s designs.

In its memos outlining stepped-up deportations, the DHS calls for the hiring of 10,000 additional Immigration and Customs Enforcement agents. The goal of vastly expanding this force is to help implement Trump’s new deportation priorities. Under the new vision, former president Barack Obama’s old approach to enforcement — in which the removal of low-level offenders and longtime residents was de-emphasized, in order to focus limited enforcement resources on serious criminals, and recent and/or serial border crossers — has been scrapped.

Now, under Trump’s newly created enforcement priorities, many more millions of undocumented immigrants are considered targets for removal. But, as a former acting head of ICE argued to me, the risk is that, if the current deportation machinery expands its focus, that could divert resources from focusing on just the serious criminals, potentially making us less safe. So expanding the number of ICE agents might be necessary to seriously expand the population of people targeted for removal, and this is what DHS is trying to do.

But Congress has to pay for that. Estimates vary on the cost of hiring, training, and equipping 10,000 ICE agents, but a recent Politico piece estimated the cost to be as high as $4 billion a year. (Politico also reported that there are serious logistical challenges in ramping up that force.) And so, the question of whether to appropriate the money for this stepped up deportation force is likely to become a major flash point in coming budget fights.

Democratic aides I spoke to today said they see a chance to mount a major stand against Trump’s expanded deportations, by opposing the enormous additional expenditures that will be required to hire all those additional agents.

It’s unclear how or when Republicans might try to appropriate the billions needed to pay for Trump’s expanded deportation force. But at some point, Republicans are going to have to decide whether to do this, and if so, how. The problem is that Republicans are already struggling to figure out how to fund another expensive piece of Trump’s immigration crackdown — his border wall. They are looking at potentially tucking the money to build the wall into a must-pass spending bill at the end of April. Do Republicans try to appropriate billions more for Trump’s deportation army on top of that?

If and when they do, Democrats can try to block that spending. Sarah Binder, an expert in Congressional procedure at George Washington University, says that any such measure is likely to be vulnerable to a Dem filibuster. Binder emails:

Because appropriations bills in any form — emergency supplemental, CR, omnibus, stand alone — typically require 60 votes to invoke cloture on the motion to call up, amend or pass the bill, 41 Democrats technically could block the spending. The unknowns are the procedural context in which the bill could come up and the particularly political dynamics that accompany it.

The political dynamics are indeed unknown. Republicans will likely argue that Democratic Senators up for reelection next year in states carried by Trump don’t dare oppose the jacked up deportation spending. But there probably aren’t enough very vulnerable senators who might be inclined to cave to get Republicans to 60. And Democrats will be under tremendous pressure from the base — and from Latinos, a core constituency, as well as Latino lawmakers — to hold the line.

Nor is it clear that Republicans have the stronger political argument. Republicans will attack Democrats for opposing additional border security. But Democrats will attack the idea of spending huge sums to forcibly evict longtime residents who don’t pose a public safety threat — they will attack the idea of spending huge sums simply to break up families. There’s no evidence that majorities want more deportations: a new CBS poll finds only 23 percent of Americans think all undocumented immigrants should be required to leave. Democrats have a strong fiscal and humanitarian argument to make in this fight.

Another big unknown is how all this is received by moderate or vulnerable Republicans in both chambers (who might not want to be associated with expanded deportations) and by libertarian-leaning Republicans who might balk at massive additional spending for a vastly expanded federal deportation force.

To be sure, it’s possible for Trump to expand deportations, at least to some degree, without all these additional ICE agents. The new deportation memos outline such moves, including recruiting local law enforcement in deportations and expanding the categories of people who can be removed without due process protections. Trump may also be content with simply expanding the numbers of people who are potential removal targets, because that could ultimately frighten many into self-deporting. But ultimately, it’s unclear how far Trump can get in actually expanding deportations without expanding his deportation army. It’s also unclear that he’ll get that expanded army as easily as he might think.