Former British prime minister Boris Johnson addresses Conservative Party supporters at the National Army Museum in London on July 2, 2024. (Justin Tallis/AFP via Getty Images)

(LONDON) -- As former British Prime Minister Boris Johnson addressed a Conservative Party rally in London on Tuesday, he made clear that opinion polls were predicting his party was expected to be swept out of power with Thursday's election.

"I think and hope the British people will show more sense on Thursday and draw back from the brink," Johnson said during his first campaign speech in support of current Prime Minister Rishi Sunak.

One of Sunak's cabinet members, Home Secretary Suella Braverman, put the party's prospects more succinctly the following day, writing in an op-ed published Wednesday that "it's over."

After 14 years in power with five consecutive prime ministers -- including Liz Truss, who served just 44 days -- the Conservatives, or Tories, are expected to be voted out in favor of Labour, a center-left party that has run on a platform calling for "change."

Polls opened at 7 a.m. across the United Kingdom on Thursday, marking the country's first national vote since 2019. An exit poll is scheduled to be published at 10 p.m., suggesting winners in key races.

Opinion polls in Britain have for more than two years predicted the Conservatives are heading for a "heavy defeat," Pippa Catterall, a professor of history and politics at the University of Westminster, told ABC News.

"The Conservatives nonetheless went into the election with the hope that, as usual, the polls would narrow in their favor," Catterall said. "Instead, it got worse."

The Labour Party's leader, Keir Starmer, who would become prime minister, said he hoped to put an end to the Conservative "chaos."

His campaign released a "manifesto" laying out some of his government's plans, including cutting hospital wait times, ushering in a greener economy and improving the U.K.'s post-Brexit trade deals with the European Union.

"From Ukraine to Gaza, Labour's foreign and defense policy essentially mirrors the current government's," Dr. David S. Moon, senior lecturer in politics at the University of Bath, told ABC News. "Starmer's Brexit plan lacks specifics beyond promising to negotiate a 'better deal.'"

Moon and other political observers said it's little surprise the Labour Party has focused its messaging on what the Conservatives have done with their time in office rather than campaigning solely on what they hope to accomplish.

Since David Cameron's victory in the 2010 general election, the center-right Tories have led the country through a period of economic "austerity," its exit from the European Union, the COVID pandemic and the current cost of living crisis.

"A Starmer victory seems more the product of a media backlash against the Conservatives following the scandal-plagued Johnson and Truss years rather than excitement about policy proposals, with Labour now seen across newsrooms as a safe alternative following Starmer's shifting of the party to the right," Moon said.

Cameron had called for the Brexit referendum before stepping down from power in 2016. He was followed in office by Theresa May, who put together a Brexit deal but stepped down before the deal could be completed. Johnson in his 2019 campaign promised to "Get Brexit Done," and the country left in January 2020.

But the Tory's leadership during the U.K.'s messy exit from the European Union is likely only one of many reasons that support for the party has waned, said Sean D. Ehrlich, associate professor of political science at Florida State University.

"The Conservatives are deeply unpopular in large part because they are no longer viewed as competent," he said. "Their struggles to implement Brexit play a large role in this decline in support but the election should probably not be viewed as a repudiation of Brexit itself."

About six weeks ago, Sunak stood in the pouring rain in front of 10 Downing Street, the prime minister’s official residence, and announced he was calling an election.

The prime minister, who came to power in 2022 via an internal party election, has said Labour's election would lead to weaker border security and higher taxes. He said Thursday that a Tory defeat could mean "higher taxes for a generation."

"Labour would increase taxes on every part of your life, including your death," he said on social media.

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