Tsongas said his supporters must not be forgotten as the campaign progresses.
"The fact is the Democratic Party's going to lose unless it speaks to exactly the constituency that I spoke for," Tsongas said. "That is a fact."
Tsongas also commented on possible independent candidate H. Ross Perot, the Texas billionaire who is contemplating a run in November. Perot, like Tsongas, has called for pro-business policies and an end to government waste.
"The fact is that Ross Perot's argument is not dissimilar from my own," Tsongas said.
Still, Tsongas questioned whether the founder of Electronic Data Systems had the patience to run for president.
"Ross Perot is a CEO who is used to people doing what he asks like that," Tsongas said, snapping his fingers.
Tsongas' announcement ended a campaign plagued first by lack of attention, then by a lack of funds. Tsongas won several Democratic primaries, including the first-in-the-nation contest in New Hampshire, but was crippled by Clinton's Southern strength on Super Tuesday and defeats in Michigan and Illinois one week later.
The candidate will carry 520 delegates, as well as any forthcoming ones, to the convention. At present, Clinton has 1279 delegates, and Brown has 272