Lamont began showing signs of considering a run against Lieberman around February 2005. In March 2006, Lamont officially announced his campaign for the United States Senate against Lieberman. As of July 19, 2006, Lamont had spent over $2.5 million of his own personal fortune on his campaign.[23] It was reported that as of September 11 Lamont had spent another $1.5 million of his own money on the campaign, with two months yet to go. [24] Lamont continued to pour personal funds into the campaign during September, with reports indicating his total contributions now exceed $12.7 million.[25] Also, Lamont pledged not to take money from lobbyists.[26] Lamont's campaign manager, Tom Swan, was, however, a registered lobbyist with the state of Connecticut. [27] In early 2006, Lamont received the backing of former independent Connecticut Governor and Republican U.S. Senator Lowell Weicker, who was unseated by Lieberman in 1988.[28]
Lamont eventually portrayed himself as an anti-war candidate calling for an immediate withdrawal of U.S. forces from Iraq, in contrast to Lieberman, who supports the continued presence of U.S. troops in Iraq and advocated for a troop increase proposed by then-President George W. Bush.[29] His campaign was partially supported by anti-war activists who oppose the Iraq war and are calling for an immediate withdrawal from Iraq, including MoveOn.org, which donated $251,156 from its contributors to the campaign.[30][31]
On July 6, 2006, Lamont faced off against Lieberman in a 51-minute televised debate which covered issues ranging from the war in Iraq to energy policy to immigration. [32] Lieberman argued that he was being subjected to a "litmus test" on the war, insisted that he was a "bread and butter Democrat" and on a number of occasions asked, "Who is Ned Lamont?" Lieberman asked Lamont if he would disclose his income tax returns. After the debate, Lamont did release his 2005 tax return and financial details about prior years. Lamont focused on Lieberman's supportive relationship with Republicans ("...if you won't challenge President Bush and his failed agenda, I will") and criticized his vote for the "Bush/Cheney/Lieberman energy bill." Lieberman stated, in response to Lamont's assertion that he supported Republican policies, that he had voted with the Democratic caucus in the Senate 90% of the time. However, Lamont argued that the then three-term incumbent lacked the courage to challenge the Bush administration's handling of the Iraq War.[32]