David Cameron will enter the fray for the first time in the hotly-contested Eastleigh by-election battle.
The Tories are hoping to take the seat from their Liberal Democrat coalition partners in a contest triggered by the resignation of disgraced ex-Cabinet minister Chris Huhne, following his conviction for evading speeding points.
Nick Clegg has already made his first foray into the Hampshire constituency, with further campaign visits expected - underlining how high the stakes are for both leaders.
A win for the Conservatives would help to convince restive Tory backbenchers that Mr Cameron still has a chance of leading them to outright victory at the next general election.
Equally Mr Clegg needs the Lib Dems to hold the seat if he is to reassure supporters that they are not facing a catastrophic collapse the next time the country goes to the polls.
In the Commons on Wednesday, Mr Cameron praised the Tory candidate Maria Hutchings as a "straight-talking" campaigner who would make "a great Member of Parliament".
Ms Hutchings has however already ruffled feathers in the Conservative high command by speaking out against Mr Cameron's plans for gay marriage and suggesting she would vote for Britain to leave the EU.