It's not that peers are trying to block Brexit. They have been adamant, time and again, that's not their game.
But tomorrow they are voting on a change they are trying to make to the government's Article 50 law (the legal process that will start Brexit), by pushing the government to give guarantees about the future of the three million or so people from other countries in the EU who live here.
Time and again Theresa May has said that she wants them to have "reciprocal rights" - to be allowed to stay and live as they wish - as long as Brits abroad get the same entitlements.
And time and again, she has said she wants to settle the issue early in the Brexit negotiations, and is hopeful of doing so.
Ministers have stopped short of giving a firm guarantee though. That's what peers want, and a Labour amendment tomorrow with support from peers from all parties will push for exactly that. And it seems there is enough support in the House of Lords to back the plan. That means defeat for the government, and embarrassment for Theresa May.
This afternoon, the Home Secretary, Amber Rudd, has written to all members of the House of Lords in an effort to reassure. In the letter, she says that "nothing will change for any EU citizen, whether already resident in the UK or moving from the EU, without Parliament's approval".
The letter is not that different to what was sent to MPs previously to try to ease their minds, as the Article 50 legislation made its way through the House of Commons. It does though appear to kill off the idea that Theresa May will arbitrarily set a cut off date for EU immigration without having to get MPs or peers onside first. But it is unlikely to spare the government's blushes tomorrow. Without a further more dramatic concession, they are set to lose.
That will set in train the first 'ping' of the potential 'ping pong' - the Parliamentary process where the Lords reject something in the red chamber, sending it back down the corridors to the green benches - daring, imploring perhaps, backbenchers to join with them and push back at the government.
There is no sign at the moment that ministers want to budge on this issue. There's frustration that they have so far not been able to resolve it with their EU counterparts.
One cabinet minister told me it's "the Germans standing in the way". They are refusing to discuss anything at all until the formal process is underway.
'Not that bad'
A senior EU source though suggested this could have been dealt with already if Number 10 had played it differently. They argued, if Theresa May had tried to resolve the issue first with European Council president Donald Tusk on behalf of the whole of the EU, there could have been an early deal.
But by going to Germany first, seeming to try to deal country by country, it would never have been possible for Frau Merkel to agree. The source suggested this was an example of how Number 10 is yet to understand fully how much the dynamics of the EU dictate that the other 27 will try to stick together. Better handling of that could, perhaps, have avoided tomorrow's likely defeat.
Yet a predictable loss in the Lords may in one way be no bad thing for the government at this early stage of the whole process. The Lords aren't going to block Brexit, but they do want to make their voices heard.
And a source suggested that tomorrow's defeat "is not that bad for either side". Once the legislation makes it back to the Commons, if there are only a handful of Tory backbenchers who pile in on this issue, the government may well be able to ride it out. But that is a big if.
Ministers are well aware that tomorrow's defeat may well be the first of many. For a long time there have been warnings that Brexit is going to get pretty bumpy for the government in Parliament.
It's true that Theresa May has had to give a few concessions here and there to avoid defeat but tomorrow, for the first time, the government is likely to be beaten over Brexit. Not by MPs, but by the House of Lords. Sources tell me they don't just expect ministers to be beaten, but for the opposition to "win handsomely".