The U.K. House of Lords began debating the draft law that would allow Prime Minister Theresa May to trigger Britain’s departure from the European Union, with some members seeking to make changes that opposition lawmakers failed to secure in the lower House of Commons.

In an unusual move, May sat on the steps in front of the Royal Throne. Though silent, her presence reinforced a warning to the unelected chamber not to meddle with her plan to trigger Brexit by the end of March. Earlier on Monday she said the Lords should “pay attention” to the fact that the bill was passed unamended by the House of Commons.

“We hold the house of Lords in the highest regard and are great respecters of its constitutional right to perform the role it does,” her spokesman, Greg Swift, told reporters in London when asked if the prime minister was there to intimidate lawmakers.

So far 30 amendments have been proposed. While that’s nowhere near the more than 250 submitted in the lower house, May’s lack of a majority in the Lords means she’s more vulnerable to losing there. In particular, some members are seeking a vote on May’s eventual Brexit deal that would leave time for her to return to her EU counterparts and negotiate changes if Parliament finds the accord unsatisfactory.

“We will not block, wreck or sabotage the legislation before us,” the main opposition Labour party’s leader in the Lords, Angela Smith, told lawmakers. “Whatever our personal views, disappointment and genuine concern for the future, that’s not the role of this House.”

Ping Pong?

The general debate on the bill takes place on Monday and Tuesday, with discussion on substantive changes not due until next week. If the Lords change the bill, it will return to the Commons, in a process known as ping pong where the bill bounces back and forth until both chambers can agree.

The Brexit Bill “comes to us with a strong mandate from both the people and the elected house and we should not overlook that,” the government’s leader in the upper house, Natalie Evans, told lawmakers on Monday. “This bill is not the place to try to shape the terms of our exit” or attempt to “re-run the referendum,” she added.

Evans said 187 lawmakers in the upper house are due to speak in the debate.

The demands for parliamentary scrutiny and a call for the Conservative premier to guarantee the rights of more than 3 million EU nationals living in Britain are the areas where May is most vulnerable to defeat in the Lords.

“There’s a strong body of opinion across parties and amongst the independent peers as well that both these issues are very serious,” Peter Mandelson, a Labour upper-house lawmaker and former EU commissioner, said on Sunday in a BBC TV interview.

May staved off a rebellion in the Commons by promising lawmakers a vote on the final agreement, which Brexit Minister David Jones said would be a choice between leaving the EU with the negotiated deal or no deal whatsoever.

Some lawmakers have said they want the government to go back to the negotiating table in the event of losing the vote, an idea rejected by Jonathan Hill, another former EU commissioner who sits in the upper house for the Conservatives.

“The idea that at the end of that process of negotiation, a British negotiator says, ‘Oh, I’m terribly sorry, that deal I’ve just offered you and struck with you, I’m afraid that Parliament have just voted on it and changed the terms’ — I don’t see how we can negotiate in good faith if we were to proceed on that basis,” Hill told BBC Radio on Monday.