LONDON — The results of the latest parliamentary by-elections in Britain cast a dark shadow over the U.K. Independence Party, once the new kid on the block.

UKIP did not make a meaningful advance in either contest. In the northern seat of Copeland, which the incumbent Conservative Party took from the opposition Labour Party, UKIP was not even competitive, barely saving its deposit with 6.5 percent and seeing its share of the vote fall back by 9 points.

The more important battle took place in Stoke Central, an area controlled by Labour since the 1930s and where UKIP had pinned its hopes on causing a major upset, as part of a push to court the Labour vote.

And yet the radical right party fell well short: As it did in the 2015 general election, UKIP finished second, increasing its share of the vote by only 2 points to 24.7 percent and leaving the party and its new leader Paul Nuttall more than 12 points behind Labour. It was bad news for the insurgent party, which thrives on creating waves not treading water.

The result is especially disappointing for UKIP for two reasons. First, this by-election took place as the Labour Party has fallen to 24 percent in the polls and its leader, Jeremy Corbyn, suffers the lowest ratings on record for any leader of either of the big parties. Despite Labour's poor prospects, UKIP was still unable to significantly increase its vote.

Second, Stoke Central was — on paper — ideal territory for the party. Located in the West Midlands, a region that has traditionally supported nativist and populist campaigns, Stoke is filled with the white, working-class, pessimistic and economically left-behind voters who had initially propelled UKIP to the forefront of British politics and overwhelmingly voted for Brexit last June. Stoke recorded the highest support for Brexit of any city in the U.K. in June's EU referendum and was subsequently dubbed the "capital of Brexit."

A declining industrial heartland best known for its potteries, this seat offered up a perfect cocktail of economic stagnation, political disillusionment and an instinctively socially conservative population when it came to issues like immigration and EU membership.

UKIP's failure to capture this seat from Labour should be set against the fact that nearly 40 percent of Stoke voters backed UKIP in the 2014 European Parliament elections, up from 22 percent in 2009. Two years later an estimated 65 percent of voters in this seat opted for Brexit. No wonder then that when the incumbent Labour MP, Tristram Hunt, resigned to become director of the Victoria and Albert Museum Paul Nuttall saw an opportunity for his party to regain momentum.

But UKIP squandered its opportunity.

Since the Brexit vote, which for UKIP marked the culmination of more than 20 years of campaigning, the radical right party has struggled to develop a new offer for those who have already voted for Brexit. In national opinion polls UKIP's average share of the vote has dropped to around 10 percent, while more broadly the party's message is far from clear. Many voters have been left wondering what UKIP stands for, now that Brexit is firmly on the horizon.

UKIP's campaigns and performances at recent parliamentary by-elections have been flat. The passion and drive that characterized the party's earlier efforts during the 2010-2015 parliament, notably ahead of its 2014 by-election victories in Clacton and then Rochester and Strood, have all but disappeared.

These problems became particularly obvious last December, six months after the referendum, when UKIP fought a by-election in the northern seat of Sleaford and North Hykeham, located in the anti-EU heartland of Lincolnshire. The party's post-referendum strategy of pinning its future on exploiting public fear over a thwarted Brexit didn't pan out. UKIP's share of the vote fell by two points to 13.5 percent.

UKIP's woes make sense, partly, within the U.K.'s new political landscape. Former UKIP leader Nigel Farage led the party at a time when the traditional third party, the Liberal Democrats, were locked into a coalition government while then Prime Minister David Cameron offered the country a more socially liberal conservatism. The party thrived because it could make serious inroads among social conservatives — voters who wanted "none of the above" and disillusioned blue-collar workers who tended to favor Labour in the past.

But the vote for Brexit and the arrival of Prime Minister Theresa May put a wrench in UKIP's progress. May's high-profile and unwavering support for Brexit, selective education and her clear desire to prioritize immigration reform in the forthcoming negotiations resonated strongly among those who might otherwise have remained in, or joined, the UKIP camp.

Meanwhile, the revival of the Liberal Democrats, now outside of government, brought a new alternative for voters who fancied a protest vote.

UKIP thus found itself squeezed as May closed off the Brexit front and appealed directly to aspirational middle England and blue-collar workers. Along the way, it was the Conservative Party that became the most trusted party on immigration, Brexit, the economy and almost every other issue, returning to over 40 percent in the polls.

This development also reflects UKIP's failure to develop a wider message beyond Brexit and immigration. Focusing on these issues in the era of David Cameron made good strategic sense. But in the era of Theresa May, UKIP's voter appeal is being overshadowed by a far more compelling offer — an incumbent government that appears to take their identity concerns, values and aspirations seriously.

So what now for UKIP? The result in Stoke Central is undoubtedly a disappointment for the party and raises fresh questions about whether it can survive over the longer-term and ever overcome the British first-past-the-post system.

But the fact that it finished second in a Labour heartland with nearly 25 percent of the vote underlines that UKIP cannot be completely discounted. Nationally, there is still a core UKIP vote, although it does not extend much beyond 10 percent. The big risk for the party now is that many of its voters will defect to the Conservative Party after concluding that Nuttall and his party are a busted flush.

Everything will depend on how UKIP donors react. Arron Banks, the party's maverick funder, has frequently made clear his desire to launch a broader movement that is capable of mobilizing a Trump-style populist revolt against Westminster. Whether or not he proceeds with this plan remains to be seen. Today, only one thing is clear: The future of UKIP is more uncertain than ever.

Matthew Goodwin is professor of politics and international relations at the University of Kent and a senior fellow at Chatham House.