Boris Johnson has pleaded with Brexiteers not to back the Brexit Party in a crunch by-election.
The prime minister faces having his majority cut to just one if the Conservatives fail to hold on to Brecon and Radnorshire.
Ex-MP Chris Davies lost the seat after being ousted by constituents following his conviction for submitting fake expenses invoices.
The Tories have backed Davies to fight the seat again despite his conviction.
Johnson told voters: “The Brexit Party cannot deliver Brexit, only the Conservatives can.”
He said the Lib Dems “will do everything they can to stop Brexit” so “if the people of Brecon and Radnorshire want to get Brexit delivered on time, they need to vote for my candidate, Conservative Chris Davies”.
During his visit to Wales, Mr Johnson faces a difficult meeting with first minister Mark Drakeford who said he would give him a “very clear message” that “Brexit will be catastrophic for Wales”.
“It will decimate our agricultural and manufacturing sectors and risks ripping the union apart,” he said.
“The PM must stop playing fast and loose with our country.”
Johnson was using the visit to highlight support for farmers, claiming they will be boosted by leaving the EU’s common agricultural policy and by the UK signing new trade deals.
But farmers are unlikely to be assured by the Welsh secretary after he offered a car-crash interview on BBC Radio 4 earlier in the day.