The campaigners from Extinction Rebellion UK blocked the bus on Tuesday (December 10) - just two days before voters go to the polls - after Johnson had held an event at a factory in central England.
They said they were calling for a faster response from Johnson's Conservatives to the climate emergency.
"The Conservatives have a decarbonisation target of 2050 which is quite frankly a death sentence for everybody," said one of the protesters, James Moulding.
A Conservative Party spokesman was not immediately available for comment.
Extinction Rebellion has previously targeted campaign buses from the opposition Labour Party, Liberal Democrats and Brexit Party.