Theresa May and Jeremy Corbyn will be given an indication of their General Election prospects as results from local contests across Great Britain are declared today.
Experts have forecast a difficult set of elections for Labour, with former heartlands in Scotland and Wales particularly vulnerable.
The Conservatives will be interested in council results in Westminster's swing seats ahead of the June 8 vote, with Mrs May hoping her gamble of calling a snap election will boost her majority in the Commons.
Votes have been cast in elections to 88 councils in England, Scotland and Wales, plus mayoral contests which include six new devolved super-regions.
As well as the looming General Election, the local contests will also be the first chance for many to express their views on Brexit, with the Liberal Democrats hoping to pick up votes as a result of their pro-Remain stance.
The Tories hope Mrs May's handling of the process will attract support from Labour and Ukip, which had a breakthrough night in England in the equivalent English local contests in 2013.
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, in a Political Studies Association briefing, also suggested the Lib Dems could gain seats in England while saying Ukip was facing near wipe-out.
Only some areas of England and Wales were counting overnight and none of the ballots in Scotland will be counted until Friday.
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 hopes 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.