The general election provides a chance to choose between different parties.
But what voters think of the party leaders - the candidates for prime minister - can also matter when they decide how to cast their vote.
So what do voters make of the leaders of the main parties fighting seats across Great Britain at this election? And how does their popularity compare with the leaders in 2017?
What do voters make of leaders in the 2019 election?
In recent weeks, the polling company Opinium has asked voters whether they approve or disapprove of the job that each party leader is doing - or, in the case of Boris Johnson, how they think he is performing as prime minister.
None of the four leaders (comparable figures are not available for the SNP's Nicola Sturgeon) emerges as especially popular.
The least unpopular is Boris Johnson - the proportion of voters who approve of the job he is doing is more or less equal to those who disapprove.
In the case of the other party leaders, those who disapprove clearly outnumber those who approve.
However, in the case of Liberal Democrat leader Jo Swinson, many voters say they neither approve nor disapprove - an option chosen by 39%.
What do party supporters think of their leaders?
As one might expect, party leaders are typically much more popular among those who say they are going to vote for the party they lead.
This may be because they support the party anyway and are inclined to approve of whoever is in charge.
Others may have been persuaded to vote for a particular party because they like the current leader.
Against that backdrop, it is notable that Nigel Farage is less unpopular than Jeremy Corbyn. This is the case even though, as a party, many more people currently say they will vote Labour (29%) than for the Brexit Party (8%).
One reason for this is that Mr Corbyn is less popular among his party's supporters than any of the other party leaders are among theirs.
Among Conservative supporters, 86% approve of the job that Mr Johnson is doing; 82% of Brexit Party supporters say the same about Mr Farage, while 77% of Liberal Democrats approve of Ms Swinson.
However, just 63% of those who say they intend to vote Labour approve of Mr Corbyn. A significant minority, 16%, say they disapprove.
How does Brexit affect support for party leaders?
It is notable that supporters of each of the two parties in favour of leaving the EU are inclined to approve of the leader of the other Leave-supporting party.
As many as 40% of Conservative Party supporters approve of Mr Farage, while only 16% disapprove. Mr Johnson receives the approval of two-thirds of Brexit Party supporters (66%), while only one in six (16%) disapproves.
However, in both cases, there has been a gradual decline during the last month in the proportion expressing approval, perhaps because voters have become more partisan. It remains to be seen whether Mr Farage's decision not to contest seats currently held by the Conservatives will reverse that trend.
Are leaders more popular than at the last election?
But how does the popularity of the party leaders now compare with the early stages of the 2017 election?
There are some striking differences.
The then Prime Minister, Theresa May, began the campaign with almost half of voters (48%) saying they approved of her leadership. Just three in 10 said they disapproved. Boris Johnson has started the 2019 campaign with much less of an advantage (39% approve, 40% disapprove).
Jeremy Corbyn, in contrast, began the 2017 campaign with a high level of disapproval. Indeed, only just over half (54%) of those who were backing Labour said they approved of his leadership - even lower than the 63% who do so now.
However, among voters as a whole, Mr Corbyn was a little less unpopular than he is now, with 53% saying they disapproved of his leadership compared with 58% now.
In short, the leaders of the two largest parties are less popular at the start of the 2019 election campaign than at the same point in 2017.
In contrast, Jo Swinson is a little less unpopular than Tim Farron, the Lib Dem leader at the last election. And Nigel Farage is much more popular than Paul Nuttall was as leader of UKIP - the party that was previously the principal advocate of a Eurosceptic outlook.
So, the leaders of the two smaller Great Britain-wide parties begin this campaign in a better position than their equivalents did in 2017. In both cases their party is more popular than it was two years ago.
How popularity can change during an election
The 2017 election saw a substantial change in the popularity of the leaders.
Theresa May became less popular as the campaign went on, while Jeremy Corbyn became more popular.
As the campaign drew to a close, only slightly more voters (44%) said they approved of the then PM than said that they disapproved (39%). While most Conservative voters retained their faith in her, the respect that Mrs May once enjoyed among some voters who supported other parties had largely disappeared.
Evidently, leaders cannot assume that any initial popularity will survive the rigours of an election campaign.
Meanwhile, by the time voters went to their polling station in 2017, only slightly more (42%) disapproved of Mr Corbyn's leadership than approved (35%). Labour voters, who became more numerous during the campaign, became quite enthusiastic about him.
Mr Corbyn will be hoping to achieve a similar transformation in his image a second time around.
Meanwhile, the other party leaders will be doing their best to ensure that history does not repeat itself.
About this piece
This analysis piece was commissioned by the BBC from an expert working for an outside organisation.
This piece uses Opinium polling on the leaders of the parties competing in the general election across Great Britain. Comparable results for parties with candidates in individual nations, including the SNP, are not available.
Edited by Duncan Walker
Charts by Mike Hills