Theresa May has been Britain’s prime minister since June. (Pool photo via EPA)

British Prime Minister Theresa May on Tuesday called for an early snap election. (Unlike in the U.S. system, the United Kingdom’s parliamentary system enables the ruling party to call for elections before the end of a term.) The call came as a surprise, as May said she would not call an early election when she became prime minister less than a year ago.

But calling early elections is not uncommon in Britain. On Wednesday, the British Parliament voted in favor of the elections, launching the six-week election campaign, with balloting to be held June 8.

Looking for a mandate

Theresa May became prime minister after voters approved Brexit — the British referendum to leave the European Union — in June 2016. The previous prime minister, David Cameron, had campaigned against leaving the E.U., and when Brexit passed, he resigned.

May is seeking a personal mandate to lead her country through the coming Brexit negotiations with the European Union. Currently, her majority is slim: Her Conservative Party holds 330 out of 600 seats, and May contends that opposition parties have been trying to thwart her efforts to leave the E.U. If she can increase her majority, passing Brexit legislation will be easier, even if there is dissent within her party.

Why this can be done

Presidential systems often have a fixed amount of time between elections. The executive (president) and legislature (parliament or congress) are chosen separately.

In parliamentary systems, only the legislature’s governing party is chosen on election day. In Britain, the leader of the majority party becomes prime minister, the British chief executive. Last year, after Cameron resigned, the governing Conservative Party selected May to be prime minister.

Parliamentary systems often don’t have fixed terms. Most parliamentary systems include a mechanism to “dissolve” parliament and call early elections. This often occurs when a prime minister can no longer pass major legislation with the support of a majority of lawmakers.

The maximum term for a British prime minister is five years. In 2011, parliament passed the Fixed-Term Parliaments Act. This set the election date to be the first Thursday in May every five years, beginning in 2015. If early elections are called, then the next election resets to the first Thursday in May five calendar years later.

When she became prime minister last June, May said that she had no intentions of calling an election before 2020. But she is within her rights to call one early — the Fixed-Term Parliaments Act allows for early elections when two-thirds of parliament agrees.

The opposition Labour Party went along with the measure for new elections, giving May the two-thirds majority she needed. Labour will campaign on a softer Brexit, education, health, and income inequality. A soft Brexit might involve trying to keep Britain in some European Union institutions, such as the single market, without having to follow all E.U. laws on other issues.

How British PMs have used early elections since World War II

From 1945 to 1995, elections generally took place every 3.5 and 4.5 years. Prime ministers must weigh the benefits of calling early elections against the costs of incurring voters’ distrust. A government might call an election early if it knows the ruling party will not perform well in a later vote. Voters are skeptical of such “political surfing.”

Consider the strategy of Margaret Thatcher, who became prime minister on May 3, 1979. Her first three years produced unpopular economic policies and high unemployment. By the fourth year, the economy had recovered, and citizens had gained confidence in her governing ability after Britain won the Falklands war in the spring of 1982. But had she called the election just after that war, “surfing” on the Falklands victory, voters might have raised their eyebrows. She held off elections until June 9, 1983 — and voters rewarded her Conservative Party with an additional 38 seats.

By contrast, Labour’s Gordon Brown become prime minister after Tony Blair resigned in 2007. While popular, he chose not to have an early election — and soon after, Britain, like the rest of the developed world, succumbed to the Great Recession. Brown was defeated in 2010. His decision not to seek a personal mandate in an early election has been viewed as a great mistake.

Will it work?

Teresa May does not want to repeat that mistake — and this is an excellent time to call for elections. Economists just upgraded their growth forecasts for the next two years. Polls released over the weekend show her Conservatives favored by 21 percent over Labour, which is led by Jeremy Corbyn. Labour is divided and Corbyn’s leadership is unpopular; only 13 percent of those polled say he is “doing a good job” and members of Parliament are asking him to stand down from leadership.

Those certainties contrast with potential costs of waiting. Brexit negotiations are supposed to last until 2019. Scottish leaders continue to demand another referendum on remaining within the United Kingdom. Economic growth might weaken. With all this up in the air, waiting until 2020 for an election might be disastrous. An election now would push the next election to 2022, when these issues might have resolved themselves.

Other parties have begun criticizing this early election. Scottish Prime Minister Nicola Sturgeon said May’s act was “for selfish, narrow, party political interests.” Shadow Chancellor John McDonnell has already suggested that voters should distrust the call, saying that “the economy is going to turn, we are seeing inflation increasing, wages stagnate and people in heavy debt. They know . . . they’ll be deeply unpopular.”

If the opposition convinces voters that May is trying to manipulate them, early elections may backfire. If not, there may be a big payoff. Recent research suggests that, on average, when prime ministers call early elections to take advantage of a divided opposition and supportive public, they gain eight percentage points and have a 26 percent greater chance of remaining in office.

May’s likely victory here would give her the mandate to insist that British citizens support her efforts to extricate Britain from the European Union.

Matthew E. Bergman is a lecturer at University of California at San Diego whose work focuses on comparative politics and political economy, especially in Europe.