Scotland's first minister, Nicola Sturgeon, speaks in the Scottish Parliament on Wednesday during the first day of debate on a motion to seek the authority to hold an independence referendum. (Andrew Cowan/AFP/Getty Images)

The Scottish Parliament will vote Wednesday evening on whether to back calls for a second independence referendum, potentially setting the stage for a clash between the British prime minister and the first minister of Scotland.

If the semi-autonomous Scottish Parliament does vote in favor of a referendum — as is widely expected — it will mean that advocates for Scottish independence will be able to say they have a parliamentary mandate for a referendum. But they will still need to get the green light from Westminster in order to hold a binding referendum.

During a debate on the issue at Holyrood on Tuesday, Nicola Sturgeon, the first minister of Scotland, said that it would be “unfair and utterly unsustainable” for Westminster to block her request.

Sturgeon, the leader of the pro-independence Scottish National Party, dropped a bombshell earlier this month when she said that she would seek a new independence referendum.

In 2014, Scotland voted 55 percent in favor of staying in the United Kingdom. At the time, Scottish leaders said it was a “once in a generation” vote. But Sturgeon has argued that last year’s E.U. referendum — which saw the majority of Scottish voters opting to remain in the E.U. and the majority of English voters opting to leave — represents a “material change in circumstances” and that Scotland risks being taken out of the E.U. against its wishes.

The timing of the proposal has come under close scrutiny. Sturgeon has suggested the referendum be held sometime between the autumn of 2018 and the spring of 20019, creating a dizzying scenario that could see Britain engaged in Brexit negotiations and an independence referendum at the same time.

Downing Street hasn't refused the request for a referendum, but it has rejected the timetable.

“Now is not the time,” said Britain’s prime minister, Theresa May, who next week will trigger Article 50 and start the process of leaving the E.U. Divorce talks between the U.K. and the E.U. are expected to last two years.

Sturgeon responded saying she’s prepared to negotiate on the timing of a referendum “within reason.”

Pollsters say that support for independence isn’t radically different from where it was in 2014, when 45 percent of voters favored independence.

“The truth of the matter is, we’re split down the middle on whether we want a referendum, and on the outcome itself,” said Mark Diffley, research director at Ipsos MORI.

When the last referendum campaign kicked off in 2012, support for independence hovered around 30 percent. At the time of the vote two years later, it had climbed to 45 percent.

“One of the things the SNP and the yes side will be banking on in any campaign is that that will happen again,” said Diffley, who added that the pro-independence party faces the additional challenge that voters are more set in their ways.

“On the one hand, the hurdle the SNP has to get over is much smaller this time around, but on the other, opinions are slightly more entrenched,” Diffley said. “So the hurdle is smaller, but more difficult to get over.”