LONDON - Hopes for a breakthrough in the Brexit negotiations between Prime Minister Boris Johnson and the European Union have surged in recent days, but the positive mood music from diplomats masks a harder reality for Johnson: He may be forced into concessions that make his deal impossible to sell at home.
The prime minister is frantically trying to bridge a gap over the thorny issue of how to treat Northern Ireland in a post-Brexit Europe in time for a crucial summit of the European Union later this week, two people briefed on the talks said.
The down-to-the-wire talks between British and European diplomats began Tuesday morning, with some European officials predicting that the two sides would never close the divide. As the day wore on, however, the negotiators seemed to draw closer, and news of a potential deal trickled into the financial markets, where traders drove up the British pound.
The closed-door talks in Brussels continued into early Wednesday. In London, Johnson said nothing publicly and met privately with a parade of skeptics from his own Conservative party and unionists from Northern Ireland. There was little of the public dissent that hobbled his predecessor, Theresa May, when she undertook similar negotiations - a sign that Johnson might be on the cusp of a major step to settle Britain's three-year national drama over Brexit.
But even if he produces an eleventh-hour success, analysts said, Johnson could face a familiar conundrum in London: If he gives too much ground to Brussels on the Northern Ireland border, he will not be able to win backing for the agreement from key elements of his coalition in Parliament.
"There is a robust trade-off here," said Mujtaba Rahman, a former European Commission economist now at the Eurasia Group, a political risk consultancy, and one of the people briefed. "Technically, the deal can be done, but can the politics deliver?"
Rory Stewart, a former Cabinet minister and one of 21 lawmakers purged from the Conservative Party by Johnson over Brexit policy, was more pessimistic. "I don't think he'll get anything," he said.
On the off chance that he succeeds, Stewart said, "the deal that he will get will be significantly worse than the withdrawal agreement," referring to the treaty negotiated by May. That would leave Britain facing an economic hit and could dissuade lawmakers from supporting any new agreement.
Johnson has vowed to leave the European Union by the end of October, come what may. But after three months of political pyrotechnics, Johnson is confronting many of the same problems as his luckless predecessor, May.
Parliament passed a law obliging him to ask Brussels for an extension if he does not get a deal by Saturday or win their approval for leaving without one. Without at least the contours of an agreement in hand, he could be vulnerable to attacks by hard-line Brexiteers.
The major stumbling block in the negotiations is a familiar one: how to handle trade with Northern Ireland, which is part of the United Kingdom but shares a border with Ireland, a member of the European Union. That question has been a sticking point in the talks and continued to drive negotiations late into the night.
Under Johnson's proposal, Northern Ireland would leave the European customs union, along with Britain, but remain aligned to many of its rules as well as other European regulations. British officials have characterized it as a kind of hybrid solution, in which sophisticated technology and flexible bureaucrats would overcome the obstacle of putting new customs checkpoints on the island of Ireland.
The trouble, these people said, is translating those concepts into legal language. Britain has not yet been able to draft a legal text that would be acceptable to the European Union because of all the consequences that flow from whether Northern Ireland stays in the European customs union or leaves.
Johnson, some European officials said, needs to revert to some version of the European Union's original Brexit proposal, which would leave Northern Ireland in the European customs union at least temporarily.
But that would alienate Northern Ireland's Democratic Unionist Party, which has propped up Johnson's Conservative-led government and fiercely opposes any moves that it views as severing Northern Ireland from the rest of Britain.
May encountered similar opposition from the Democratic Unionists and offered Brussels a counterproposal under which all of the United Kingdom would remain in the European customs union for a period of time. That outraged hard Brexiteers in her party, who helped defeat her agreement three times in the House of Commons.
Johnson has so far kept the Democratic Unionists in the fold, inviting them to talks this week. But Rahman said it would be difficult for him to avoid fractures in his coalition if he agreed to further concessions with Brussels.
As negotiators huddled in Brussels on Tuesday, public statements by British and European officials remained cautiously upbeat. A deal "is still possible this week," said the chief negotiator for the European Union, Michel Barnier, though he urged the British to submit a legal text.
Some of Johnson's anti-Brexit opponents fear he might reach an unexpected breakthrough Friday, then pressure Parliament to approve the deal in a rush Saturday, the deadline for requesting an extension.
In addition to the legal questions, there is the sheer complexity of Johnson's customs plan, the issues it raises in terms of tracking the destination of goods and the risk that it would encourage smuggling.
That is why even optimists say that, in the event of an agreement in principle on a deal, Johnson will almost certainly be forced to request some form of extension to the Brexit deadline.
On Wednesday Johnson will decide whether to call a Saturday session of Parliament - for the first time since a crisis over the Falkland Islands in 1982 - with lawmakers voting Thursday on whether to approve any move to do so.