There are calls for a tightening of campaign spending rules in future mayoral elections after the West Midlands' Conservative candidate is thought to have raised up to £1 million.
When former John Lewis boss Andy Street was asked if he had spent a reported £1 million campaigning, he replied: "We haven't spent quite a million - but we have spent a substantial sum."
Labour's choice to contest the region's election on May 4, Sion Simon, said he could not blame his political foe "for sticking to the rules" on spending - but said it showed changes were needed.
Under current rules, a £130,000 cap on funding only kicks in for the final weeks of the campaign, but there is no limit beforehand.
No rules have been broken, but the other five West Midlands mayoral candidates have all said they have spent far less than the Tories.
On Thursday, 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.
Asked about his campaign spending, Mr Street said: "We haven't spent quite a million pounds but we have spent a substantial sum more than my opponents and I accept that.
"This is a very important election, a new start in democracy for this region.
"It's two-and-a-half million people so, actually, the sum is absolutely appropriate.
"And all the money has been raised for this campaign - we didn't have anything at the start, so it reflects the enthusiasm."
Speaking to BBC Radio 4's Today programme, he added: "Everyone has the same rules to work within, and we were able to raise (that amount).
"But also the same rules, before that (period), says if you raise money you can spend it, and that is what we have done."
Mr Simon said: "You can't blame Andy for sticking to the rules but it does beg the question whether the rules are right, that up until six weeks before the election, there's absolutely no restriction and whoever has got the most money can spend it.
"I think they (the rules) are wrong.
"In general elections, the regulation period starts much earlier.
"No rules at all, a complete free-for-all until six weeks before polling day, I don't think is the right way to go about this."
Mr Simon has previously said that while Labour's spending may not match the Tories', it has deployed "an army of volunteers" to canvass voters.
The elected mayor will have greater powers, devolved from central government, on transport, housing and planning.
The new mayor will also chair a West Midlands Combined Authority, drawn from the area's local authorities.
Also standing in the region's election are Beverley Nielsen, for the Liberal Democrats, Pete Durnell, for Ukip, Graham Stevenson for the Communist Party and James Burn, of the Green Party.