Roger Stone, a key Republican strategist, repeated claims Mr Trump made on Twitter without providing any evidence.
"I believe that [Mr Trump] was under surveillance by the Federal government, by the intelligence agencies, while he was the Republican nominee for President," he told Russia Today, international television network funded by the Russian government. "This is a scandal bigger than Watergate."
Mr Stone, who has a tattoo of former Republican president Richard Nixon on his back, was banned from major American news networks during the 2016 electoral campaign after he used a racial epithet to describe an African American journalist.
Nicknamed "the dirty trickster", he has a reputation in Washington for underhand campaign tactics and the promulgation of conspiracy theories about his political rivals.
Mr Trump's alleged on Twitter that Barack Obama wiretapped his New York office in 2016.
He wrote: "Terrible! "Just found out that Obama had my 'wires tapped' in Trump Tower just before the victory. Nothing found. This is McCarthyism!"
In a second post, he misspelled the word "tap", writing: "How low has President Obama gone to tapp my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!"
Mr Stone supported the assertion.
He said: "This is the most outrageous breach of law and of morality in American public history. Richard Nixon did not know about the Watergate break-in in advance, but he bore ultimate responsibility.
"If you look at the statement by (former) President Obama this weekend, he said he didn’t approve eavesdropping on candidate Trump. Notice he didn’t say it didn’t happen, only that he didn’t approve it."
He also expanded on the President's reference to McCarthyism - a reference to the 20th-century American witch-hunts of suspected Communist sympathisers overseen by Senator Joseph McCarthy.
He said: "In this country, if you are not in favour of war over Syria, then you must be a traitor; you must be in the service of Russian intelligence. It’s insulting, it’s outrageous, it’s false."
He added: "To say because you are not in favour of war with Russia tomorrow you must be a traitor? This is the worst form of McCarthyism in the 40 years that I have been in American politics."
Mr Obama's spokesman, Kevin Lewis, has previously denied the suggestion that any wiretapping or surveillance had taken place.
"A cardinal rule of the Obama administration was that no White House official ever interfered with any independent investigation led by the Department of Justice," he said, shortly after Mr Trump's original allegation.
"As part of that practice, neither President Obama nor any White House official ever ordered surveillance on any US citizen. Any suggestion otherwise is simply false."