LONDON — From the purple-lit stage of a London ballroom on Monday, Prime Minister Theresa May of Britain pitched her draft deal for leaving the European Union to an audience of business leaders who, not long ago, hardly hid their distaste for the prime minister.
She had taken office two years ago complaining about corporate greed, threatening a crackdown on excessive pay and condemning free-market acolytes as “citizens of nowhere.” This summer, her then-foreign secretary, Boris Johnson, poisoned relations further when he wielded a four-letter expletive to tell businesses how little he cared for their interests.
But Mrs. May received a mostly warm reception on Monday — warmer than any she has gotten even from supposed allies in the five days since she unveiled her much-reviled plan to extract Britain from the European Union.
Her deal was, well, a deal, and it granted businesses some certainty.
That was enough for the business leaders sitting before her, though some urged the prime minister to soften proposed immigration restrictions after Britain’s split from the union.
“She put a deal on the table, which is the first time in two-and-a-half years that’s been true,” said Craig Beaumont, the director of external affairs and advocacy for the Federation of Small Businesses, who was in the audience. “Business is accepting she’s in a tough position but appreciating the progress she’s made.”
Carolyn Fairbairn, the director general of the Confederation of British Industry, said business leaders remained concerned about parts of Mrs. May’s plan, particularly restrictions that the prime minister said would keep European Union migrants from “jumping the queue” to work in the United Kingdom after Brexit.
Ms. Fairbairn said businesses worried about where new workers would come from, and rejected “the idea that anyone earning less than 30,000 pounds can’t contribute to our economy.” That amount is the equivalent of about $38,500.
How much businesses’ support will matter on these issues is a different question.
Business leaders saw their political stock sink after trying in vain to warn people of the risks of leaving the union in 2016.
Now they face having to make virtually the same pitch to conservative members of Parliament who, while sympathetic to business interests on the whole, tend to pay them little heed on the decades-old question of the United Kingdom’s relationship with Europe.
“There is an ideological attachment to Brexit that can supersede other stuff,” said Professor Anand Menon, director of the UK in a Changing Europe, a think-tank. He described it as an “issue that transcends the normal rules of politics.”
Still, Monday’s meeting of the Confederation of British Industry was a soft launch for a business-oriented campaign on behalf of Mrs. May’s deal, which will likely kick into high gear after the deal gets ratified by European leaders, as it is expected to on Sunday.
That will set the stage for a mid-December vote on the deal in British Parliament, so long as the deal survives that long.
Mr. Beaumont said his group would soon be delivering the results of a survey of small business owners to members of Parliament.
The group plans to advocate, among other things, for government vouchers allowing small businesses to seek the help of consultants as they prepare for post-Brexit trade arrangements.
With small business owners having only narrowly backed remaining in the union, Mr. Beaumont said he hoped his group would have more clout than most among politicians also needing to balance a split electorate.
“When we go into Parliament,” he said, “we’ll be giving a view that doesn’t come from either wing of this quite ideological debate.”
Imperfect as Mrs. May’s draft deal may be in the eyes of the majority of business leaders who opposed Britain’s leaving the union, it met their tests for an orderly departure, business groups have said in recent days.
It provided for a nearly two-year transition after the March split during which businesses could operate almost as if nothing had changed. It guaranteed that the entire United Kingdom would stay in a customs territory with Europe until leaders worked out a longer-term trade agreement, avoiding internal tariffs or quotas.
Above all, it offered some clarity, a point Mrs. May stressed in her speech as she eschewed the bravado of the hard-line Brexiteers in her party and appealed to practical concerns.
“Because we are not talking about political theory, but the reality of people’s lives and livelihoods,” Mrs. May said. “Jobs depend on us getting this right.”
It was a measure of how desperately businesses are trying to avoid a so-called cliff-edge Brexit — in which Britain crashes out of the union without any arrangements being made for crucial supplies to cross the border — that the audience mostly embraced her draft plan.
Few in the audience had likely forgotten their run-ins with the prime minister during her early months in office.
Mrs. May disbanded a business advisory group convened by her predecessor, David Cameron. She stepped in to fix what she took to be broken markets. The director general of the very group that hosted Mrs. May on Monday accused her in 2016 of risking “closing the door” on Britain’s open economy with a clean break from Europe.
But Mrs. May, anticipating the boost businesses would give her draft deal, has tried to shore up support in recent months by bringing in a former investment banker as a new business envoy and setting up five business advisory councils.
It helps her cause with business interests that the opposition Labour party is led by Jeremy Corbyn, who wants to nationalize some industries.
Analysts wonder, though, whether the government’s campaign of late to scare people about the calamitous economic effects of leaving the union without a deal may have proved too persuasive. With the help of some businesses, Mrs. May and her team tried to make leaving without a deal look so unpalatable that some worry people began writing off the possibility.
“They painted a picture of no-deal which is so bad that nobody credibly believes we will be allowed to fall into a no-deal situation,” said Joey Jones, a former spokesman for Mrs. May who is now strategic counsel at Cicero, a consulting firm.