Depending on your political persuasion, the next month in British politics will either be increasingly exciting or deeply depressing. The European and local elections will provide a snapshot of the fortunes of our political parties as well as Ukip's ability to fulfil its promise of "causing an earthquake".

The latest polls suggest that the party is on course to get the largest share of the vote in the European elections, becoming the first new party to triumph in a nationwide election since Labour's rise in the early 20th century. Such an outcome would represent a rejection of Britain's entire political establishment by a large section of the electorate, raising awkward questions for all the main parties.

The elections are crucial for Ukip as it seeks to establish itself as a real player in national politics. Here are the three most important questions about this revolt on the right that the elections on 22 May will help us answer.

Can Ukip broaden its base?

It is no coincidence that Ukip's election broadcast features lots of women, young people and minorities. These groups tend to shun the party. Nigel Farage is keenly aware of the problems this poses, which is why nine women appear in the party's top three spots on the regional lists from which MEPs are elected.

Since 2010, Ukip has relied heavily on a base of financially disadvantaged, old white men. There are lots of these left-behind voters, which is why Farage is hoping to "double down" on them in May, touring northern England and using billboards to target their intense concerns over immigration, Europe and "out-of-touch" elites.

But he also knows that to win Westminster constituencies he needs to reach beyond this core electorate. In 2009, Ukip managed to win a broader base of support by attracting middle-class, Eurosceptic Conservatives. However, such voters quickly lost interest in Ukip, which they saw as a tool for sending a Eurosceptic message to the Conservatives and not as a credible option at a general election.

This time, Ukip needs to retain the new supporters it attracts in its European parliament "home fixture". This is why we will be keeping a close eye on the local election results, to gauge whether the party's European voters also back them domestically. We will also be monitoring the state of the opinion polls in the summer to see if Ukip's European success is having an impact on its domestic chances.

Can Ukip identify local strongholds for 2015?

What began in the 1990s as a fringe rebellion by disgruntled Tories in the southern shires has quickly grown into the most significant challenge to the main parties for a generation. But Ukip remains hampered by the first-past-the-post system, which heavily penalises new parties with evenly spread support. To convert votes into seats, Ukip needs to build local strongholds where it can demonstrate a real ability to compete on the ground.

It is the local elections next month that offer real insight into Ukip's reach and prospects for 2015. This will be its last chance before the general election to demonstrate election-winning strength. Ukip will be looking for areas where it can top the poll and get councillors elected, because strong local results can be used as evidence that Ukip is credible locally. This is why many senior Ukippers are secretly more interested in these local battles than the European contest; they know they need another wave of local gains to have even a chance of causing an upset in 2015. For this reason, we will also be analysing the local polls: a cluster of Ukip councillors returning to town halls may well be a leading indicator of a strong Ukip challenge next May.

Can the party stand the pressure?

While Ukip is used to fending off attacks, the current wave of negative publicity is simply unprecedented in its history. Evidence of racism and allegations of expense fiddling are certainly not denting Farage's support, but they are piling on the pressure and fuelling his long-held frustration with Ukip's amateurism, as well as potentially putting off voters outside the party's core electorate who are more sensitive to concerns about extremism and incompetence.

Even more significant but ignored is the impact of all this on Ukip donors. While the likes of Paul Sykes are happy to contribute to an easier European campaign, will they be willing to back Ukip in 2015 should the party fail to get its house in order now? This is what the appointment of experienced journalist Patrick O'Flynn, a reorganisation of Ukip HQ, and tighter screening of candidates were supposed to achieve, but instead Farage has repeatedly been pushed into defensive mode. As the attacks intensify, can he steady the ship, reassure donors and finally address Ukip's achilles heel?