Democrat Ned Lamont, seeking to erase Sen. Joe Lieberman's double-digit lead, is turning to his campaign's biggest financial backer for another infusion of cash: himself.

The multimillionaire businessman, who trailed by 10 percentage points in a recent poll, wrote a $500,000 check to his campaign this week, an aide said Tuesday. Just last week, Lamont had tapped his personal wealth for $750,000 to help fund the race. He has spent $6,751,500 of his own money to unseat the 18-year incumbent.

The Lamont camp said it needs the money to blunt Lieberman's aggressive television ad campaign; both sides are spending heavily on commercials. Lieberman has run ads recently suggesting that political newcomer Lamont lacks the experience to be an effective senator.

"Ned is not going to let Senator Lieberman's negative allegations go unanswered," said Lamont spokeswoman Liz Dupont-Diehl. "We've said all along we're not going to take this lying down."

The Lieberman campaign scoffed at Lamont's move to bolster his campaign.

"We fully expect Ned Lamont to keep drawing on millions of his own money to continue funding his negative campaign," said Lieberman spokeswoman Tammy Sun.

Lieberman defied Democratic leaders by launching an independent bid after he lost the Aug. 8 primary to Lamont. The three-term incumbent, who was his party's vice presidential nominee in 2000, has built a national fundraising base. Some prominent Republicans are helping him raise money.

Lamont aides argue that he is using personal funds to finance his campaign because Lieberman is taking money from special interest groups and political action committees. Lamont has called for public financing of campaigns and lobbying reform.

Lamont spent about $4 million of his own money to win the primary. He has written $2.75 million in personal checks to his campaign since then.