The mail was sent to the home of Veritas candidate Anthony Backhouse and contained a handwritten note claiming to be from the British National Party.
Police have been called in to investigate the incident.
Detective Inspector Paul Hitchen, from Bolton CID, said they would be examining the letters for forensic evidence and finger prints.
He said: "We have got to treat this as a serious incident. The blades have been placed right inside the lip of the envelope. It does give us cause for concern."
Mr Backhouse, who is vying for a seat in the Tonge with The Haulgh ward, was warned about hate-mail on Monday by local Veritas secretary David Joneswho received an identical letter that morning.
"He said watch your post' because he was sent a letter with a razor blade under the seal of the flap. So I rang my wife and warned her not to open the post," said Mr Backhouse, of Mossfield Court, Bolton.
When he arrived home later in the day, he saw the letter.
Mr Backhouse said: "It stuck out like a sore thumb. I put on a pair of gloves and opened it and found the razor blade hidden in the flap and the note."
He then alerted the police and reported the matter to election officials.
Mr Backhouse, whose election opponents include BNP candidate David Wood, said he did not think the BNP was responsible. "It's just some crank. I am not perturbed by it in any way," he added.
The note, written on lined paper in capital letters, read: "Use your brains. Make way for the BNP."
There was also a drawing of the Union Flag.
The BNP has denied having anything to do with the letter.
Roy Goodwin, North-west regional organiser for the BNP, said: "It's absolutely ridiculous and absolutely not from the BNP.
"It's obviously from an outside source trying to discredit the BNP."
l Other candidates in the Tonge with The Haulgh ward are Elaine Sherrington, Labour, Nigel Ford, Conservative, Rosalind Harasiwka, Lib Dem, and James Tomkinson, Green Party.