Giuliani "can raise a lot of money that way, but I don't think he should be able to raise it for free," Mr. Clinton told CBS Evening News Anchor Dan Rather on Thursday.
It's the second time in nearly a week that the president has directly entered the New York Senate fray between Republican Giuliani and Democrat Hillary Rodham Clinton. Last weekend, Mr. Clinton alleged "a right-wing venom machine" was collecting money to defeat his wife.
On Thursday, the president underscored that claim, noting nationwide mailings by conservative fund-raiser Richard Viguerie soliciting money for Giuliani's campaign.
"The Viguerie mailings, which are being sent to people who have fought me the whole time I'm here (in the White House) - which is fine - are basically using the same old standard, hard-core, right-wing stuff,"
Referring to this year's South Carolina GOP primary, the president said it was "the kind of stuff we saw Governor Bush do to Senator McCain" in the Palmetto State.
The fund-raising letters on Giuliani's behalf, added Mr. Clinton, are "another way of going after us " - meaning the White House - on gun control, tax cuts and the budget surplus.
"They see that as a way of continuing the battle," he said of those sending out the letters - and those answering them with campaign contributions to Giuliani's Senate bid.
So far, Giuliani has raised $19 million for the Senate race, Mrs. Clinton $12 million.
The president said he was "a little bit surprised" by the latest CBS News-New York Times poll on the Senate race, which shows the first lady with an 8-point lead over the mayor.
"I think it's going to be a close race and a hard race. But she knows why she's running, she knows what she wants to do for New York, I'm really proud of her."
Although the poll puts Mrs. Clinton ahead of Giuliani for the first time in a year, the president said the "polls will change a lot between now and November", calling the mayor "a very formidable opponent."
And Mr. Clinton dished out a bit of campaign advice that the first lady seems to be following already.
"What she's got to think about is not what they're saying about her, but what she's going to say to the people of New York. If she'll just focus on that, talk about her life, her work and what she wants to do, I think she'll do fine."