"I have worked as hard as I could to turn our country around," the president wrote in his three-page appeal. "Now, the work must be carried on by others. No one is more qualified than Hillary to do it."
In the new Lazio ad, McCain looks straight into the camera, telling potential voters they shouldn't believe Clinton's claims that she is willing to give up unlimited soft-money campaign contributions if her opponent will do the same.
McCain said that while he at first believed the first lady would do so, he has since decided she is not really serious about banning soft money from the New York Senate race. Clinton and Lazio have been blaming each other for weeks over the failure to reach agreement on a soft-money ban.
The first lady, with the help of her husband, has been aggressively raising soft-money contributions for months to boost her Senate bid. A Hollywood gala held just before the Democratic National Convention opened in Los Angeles earlier this month raised about $1 million for her campaign.
"It's pretty clear there's only one senate candidate who really wants campaign finance reform - Rick Lazio," the senator from Arizona tells viewers. "He's the candidate you can trust."
McCain, who said earlier this year that the first lady would be a "star" in the Senate, has increasingly been lending his celebrity and his appeal to moderate and independent voters to the still little-known Lazio.
The latest Lazio ad builds on an aggressive new line of attack the Long Island Republican launched Monday when he unveiled a TV ad with the tag line, "Hillary Clinton - you just can't trust her."
Lazio said his attack is the result of weeks of TV ads from Clinton and her supporters attacking his congressional record with what he says are distortions.
"I will not be a doormat," he said during a campaign stop Tuesday.
"What we've got to do now is just counter-punch to remind the voters just exactly what they're doing," Lazio's campaign manager, Bill Dal Col, said Wednesday.
Meanwhile, President Clinton's letter "about the Senate candidate I know best," warns that his wife's opponents, "especially on the Republican right, will spare no expense and respect no boundaries in their efforts to defeat her."
The president said he was making the appeal "not just out of personal interest, but also in the hope of advancing the values and highest ideals of our party and our nation."
That's a sharp difference from what Lazio has been saying about the Clintons.
In a July fund-raising letter, Lazio told potential contributors that "Hillary Clinton and her husband have embarrassed our coutry and disgraced their powerful posts."