Theresa May and Jeremy Corbyn will be looking for any potential indicators about their General Election chances as voters have their say in local elections.
Campaigning for the national vote on June 8 will slow drastically on Thursday because of the elections at 88 councils in England, Scotland and Wales, plus mayoral contests - which include six new devolved super-regions.
But the battle for control at local authorities across the country has been overshadowed in recent weeks by the highly unusual decision - in organisational terms, at least - of Prime Minister Mrs May to call a General Election within weeks.
It also comes amid the backdrop of Mrs May accusing European politicians and officials of seeking to influence the result of the General Election, using a Downing Street statement to intensify the war of words over Brexit.
With the campaigns overlapping, candidates will wait to see if national issues such as the EU have a greater impact on their chances locally, despite the elections dealing with authorities which have different decision-making powers.
Experts last week released forecasts which predicted Mr Corbyn's Labour is heading for a "kicking" at local polls in Wales and "heading for disaster" in Scotland, with the SNP and Tories expected to make gains.
The academics, via a Political Studies Association briefing, also suggested the Liberal Democrats could gain seats in England while saying Ukip is facing near wipe-out.
Results to watch include the potential for the SNP to seize control from Labour in Glasgow, the Tories bidding to take back overall control in Norfolk and the Lib Dems depriving the Conservatives of overall control in Somerset, should they secure a 2% swing in their favour.
In the mayoral contests, former Labour cabinet minister Andy Burnham will hope to begin life after Westminster by becoming Greater Manchester's first elected mayor.
He is regarded as the favourite in the contest which will result in new decision-making and spending powers being in the hands of the winner.
A far more intriguing result is expected in the West Midlands, where two million people across Birmingham, Coventry, Sandwell, Dudley, Solihull, Walsall, and Wolverhampton will be eligible to cast a vote for the region's first directly-elected metropolitan mayor.
Former John Lewis boss Andy Street is running for the Conservatives while Sion Simon hopes to secure the role for Labour.
Some results from the various authorities will be declared in the early hours of May 5, although most are expected to come during the day.
Tory Party chairman Patrick McLoughlin, in a statement issued before voting began, drew upon rhetoric from the national campaign when encouraging support for his party's local candidates.
He added: "There is a clear choice - the competence of strong Conservative councils and elected mayors who will keep local taxes down with quality local services, versus the disarray of the rest."
Elsewhere on national issues, the Conservative Party denied reports that Mrs May is planning a Commons vote to expand British operations in Syria to include attacks on its president, Bashar Assad.
The Daily Mirror said the PM - should she win a landslide at the General Election - wants to seek approval from MPs to allow British warplanes, warships or submarines to attack Syrian regime forces in the event of further chemical attacks on civilians.
But a senior Conservative Party source said: "The story is simply not true."
MPs backed air strikes in Syria against terrorists of the so-called Islamic State in December 2015.
But the Commons rejected strikes against the Syrian government in 2013.