The purple wave is receding. Those who look to the polls for enlightenment on whether Labour or the Conservatives will be the largest party after the election will not find it. But most show one unambiguous trend: support for Ukip is in decline.

Nigel Farage’s party has been unable to sustain the giddy momentum that it acquired after it won last May’s European elections and captured two Conservative defectors. The decision of the Tories to “shut up” about immigration (in the words of one) has ensured that the campaign has been fought on less hospitable terrain for Ukip. If the state of the NHS and the economy are the defining questions, few will regard “the people’s army” as the answer. The hitherto ubiquitous Farage, meanwhile, has been largely absent from the media as he has devoted himself to the task of winning the election in South Thanet. If not burst, the party’s bubble is unmistakably deflating.

Liberals regard all of this with understandable satisfaction. Repelled by Ukip’s assortment of racists, homophobes and all-purpose bigots, they long to return to the era when the party was a political afterthought. Their wish is unlikely to be granted. To dismiss Ukip as an electoral shooting star is to underestimate both its short-term rise and its long-term potential.

If the party has peaked, it has done so at a level that few initially thought possible. Ukip continues to poll in double digits in most surveys and remains in third place (it won just 3 per cent in 2010). Many Liberal Democrat MPs privately expect to finish behind it in terms of votes on 7 May. Farage’s party will be fortunate to win more than a handful of seats; Lord Ashcroft’s recent poll of Tory-Ukip battlegrounds found it trailing in targets such as Boston and Skegness, Castle Point, North East Cambridgeshire and South Basildon and East Thurrock. Labour believes that it will lose few if any of its northern and Midlands redoubts to the party. Ukip strategists, however, are confident that they will attain a platform for greater success in 2020. They are right to be so. Whether the Conservatives or Labour enters power, Ukip will have no shortage of political ground to exploit.

Victory for Ed Miliband would most likely benefit Ukip, as some of its members privately concede. After establishing itself as the main opposition party in seats such as Heywood and Middleton (where it finished just 617 votes behind Labour in last year’s by-election), it would be the natural receptacle for discontent with the government. Miliband’s refusal to hold an in/out EU referendum and the continuation of unrestrained European immigration would ensure that its two strongest suits remained salient. Ukip’s ongoing repositioning as an economically populist party would allow it simultaneously to harness opposition to continued austerity. By framing fiscal scarcity as the price to be paid for remaining in the EU and funding migrants’ welfare benefits, it would strengthen its appeal to traditional Labour voters. And should a Miliband government be sustained by SNP support, Ukip would be gifted an opportunity to fan the flames of English nationalism.

If David Cameron remains in No 10, Ukip is similarly unlikely to drift into obscurity. The promised EU referendum would return it to centre stage as perhaps the only party campaigning for an “out” vote. As the Tories endured their greatest split since the Corn Laws, Ukip would stand to gain further defectors. Because of Cameron’s decision not to demand an end to the free movement of people, owing to German opposition, any renegotiation will not resolve the immigration question. The experience of the SNP and the Scottish independence referendum proves that defeat in such plebiscites is no barrier to further advancement. An “in” vote would be heralded as further proof of the malevolent grip of the establishment.

Should the electorate vote instead for EU withdrawal, Ukip would be deprived of its original raison d’être. It is perhaps fortunate that several polls suggest that by contaminating the Eurosceptic brand, Farage’s party may forestall this outcome. Like all insurgent movements, it thrives on struggle, rather than victory. But an end to EU membership would not necessarily result in an end to Ukip (as its early founders once assumed). The party has evolved from an anti-federalist outfit into a catch-all repository of protest. In 2007, during his first spell as leader, Farage toyed with renaming Ukip as the “Independence Party” in a bid to broaden its appeal. Whether or not it ever undergoes a similar rebranding, this title accurately describes the role it now occupies.

By far the greatest spur to Ukip success would be electoral reform, a cause embraced by Farage. Such is the potential perversity of the election result that MPs of all parties are beginning to ask whether first-past-the-post could endure in such circumstances. The introduction of proportional representation would entrench Ukip as a permanent feature of the electoral landscape.

Success is not guaranteed. The ideological tensions between the party’s multiple wings could yet overwhelm it. Douglas Carswell’s repudiation of Enoch Powell on 24 February was the clearest demonstration yet of the gulf that exists between the Ukip MP and Farage on the issue of immigration. “It can be done, but it will not be done if the argument is reduced to arguing about people from Romania,” he declared of EU withdrawal in an implicit rebuke to his leader.

Perhaps the only thing that could reverse Ukip’s advance is the attainment of power, as the Liberal Democrats learned to their cost. Once tainted by compromise, it would soon come to resemble those it assails as political swine. That is not a fate it need contemplate now. The complacent assumption that Ukip’s moment has passed is born of the same arrogance that first led to its rise.