LONDON — Her admirers see Liz Truss, Britain’s foreign secretary and a rising political star, as a potential successor to the country’s embattled prime minister, Boris Johnson — and in one recent episode, at least, she appeared to play the part. On a visit to Estonia, Ms. Truss was photographed on a tank, evoking a famous 1986 image of Britain’s first female prime minister, Margaret Thatcher, who was captured in such a pose.
Now, after a surprise cabinet resignation over the weekend, Mr. Johnson has put Ms. Truss in charge of his issues related to Brexit, his signature policy initiative. That has analysts pondering whether she has been handed a chance to secure the status of Northern Ireland and forge a coherent post-Brexit foreign policy — or a poisoned chalice.
Mr. Truss got the job after Mr. Johnson’s previous Brexit envoy, David Frost, resigned from the cabinet on Saturday, citing differences over government policy, including over the imposition of coronavirus restrictions.
With cases surging and Mr. Johnson under pressure to introduce further curbs, that issue continued to reverberate on Monday. After a cabinet meeting held on video, the government made no announcement of fresh measures, suggesting that senior ministers were pushing back against them.
But while the political crisis engulfing Mr. Johnson shows no sign of abating, his decision to give Ms. Truss new responsibilities opens the possibility of a more pragmatic and less ideological approach to an impasse with the European Union over post-Brexit trade rules. That’s especially true for negotiations over Northern Ireland, where Mr. Frost took an unyielding approach that frustrated E.U. negotiators.
“Her incentive may actually be to do a deal,” said Mujtaba Rahman, an analyst at the political risk consultancy firm Eurasia Group. “There is a big platform that she’s been afforded through her assumption of the Europe brief, and what she’s been trying to achieve in the world would be greatly facilitated by a more friendly, pragmatic, sensible relationship with the E.U.”
Others are more skeptical. Recent history suggests that Conservative Party politicians need to court hard-line Brexit supporters in Parliament and in their party in order to win the top job.
“I just don’t see how she can come in, given who she is, and make those quite subtle, quite important compromises,” said Anand Menon, a professor of European politics at King’s College London. “Frost didn’t have any particular aspirations to high office, I don’t think, whereas she does.”
Mild mannered and studiously polite in person, Mr. Frost was nonetheless the consummate Brexit hard-liner, a man who believed that only a combative approach — and a willingness to walk away without a deal — would win concessions in talks with the European Union.
He was a leading architect of Britain’s split with the bloc, negotiating a basic trade deal that took Britain out of its economic structures. He was such a close ally of Mr. Johnson that the prime minister delighted in calling him “the greatest Frost since the Great Frost of 1709,” referring to a particularly brutal cold snap more than three centuries ago. But he was not liked in Brussels, where there appeared to be little sadness about his exit.
Ms. Truss is a belated enthusiast for Brexit; she campaigned against it in the 2016 referendum. As foreign secretary, a position she assumed in a September reshuffle of the cabinet, she has pushed for new partnerships for Britain in Asia and the Pacific, more trade deals outside Europe and a tough line on Russia and China.
She does not have the same credentials among Brexit hard-liners as Mr. Frost did, and some critics questioned whether she was qualified to be foreign secretary. But conservative Britons appear to have few qualms: According to one recent survey of Conservative Party members, she was ranked as the most popular cabinet member.
Northern Ireland is a particularly fraught and high-stakes brief for Ms. Truss to inherit. Britain has threatened to suspend part of an agreement that Mr. Johnson himself negotiated, known as the Northern Ireland protocol.
The protocol was designed to avoid checks on goods crossing the politically sensitive border between Northern Ireland, which is part of the United Kingdom, and the Republic of Ireland, which remains part of the European Union.
The E.U. has threatened to impose new trade sanctions against Britain if it chooses to jettison the Northern Ireland agreement. Abandoning it would also rankle the United States, which does not want any action that could destabilize the fragile Irish peace process.
Talks on easing some of these problems have made some headway, but they were stuck over British demands to remove any role for the European Court of Justice, the European Union’s highest court, in arbitrating trade disputes in Northern Ireland. Last week, Britain seemed to soften that demand just days before Mr. Frost quit, and some believe the two events are linked.
Some analysts say Ms. Truss’s role as foreign secretary could lead to more progress in negotiations, because as foreign secretary she has to consider the wider context of international relations.
It would strengthen Ms. Truss’s more hard-line approach to Russia and China, for example, if she had a functioning relationship and better cooperation with like-minded European powers, including France and Germany.
“If she wants to achieve her objectives toward the Russians and the Chinese and to be a big player in the world and deliver on global Britain, then that actually demands a more constructive relationship with Europe,” Mr. Rahman said.
“The fact that you are now housing the Europe brief within the Foreign Office enables you to think about these questions and trade-offs under one roof,’’ he added. “That’s more likely to sponsor a more coherent policy and ultimately a policy that brings the two sides closer together.”
If Ms. Truss can win concessions from Brussels and deliver a workable outcome on Northern Ireland, it could burnish her credentials as a deal maker and enhance her status as a leading contender to replace Mr. Johnson, if he should fall.
That said, Mr. Rahman does not expect any fast results. Ms. Truss will have to prove her toughness to Brexit supporters at home and demonstrate that she is no pushover, he said.
And there is always the chance that Britain’s ties with the European Union could again become intertwined with the internal politics of the Conservative Party, as speculation swirls about replacing Mr. Johnson.
“There are two audiences for anyone who wants to become Tory leader: Tory members of Parliament and Tory members,” said Prof. Menon, “I don’t see a massive appetite among either of those groups for a face-saving compromise.”