While considering a presidential run, Mr. Chafee has shown a willingness to confront Mrs. Clinton, questioning the financial dealings of her family’s foundation and criticizing her for supporting the 2003 invasion of Iraq. Mr. Chafee, a Republican at the time, was the only senator from his party to vote against the war.

“I think Chafee wants to have the same dialogue on foreign policy that Sanders wants to have on domestic policy,” said Scott MacKay, a political analyst for Rhode Island Public Radio. “He sees the country as becoming too militaristic, and he will talk about the Iraq war vote.”

Mr. Chafee made that clear during his announcement, arguing that the Iraq war continues to haunt America’s credibility and advocating “waging peace” through multilateralism. Although he did not mention Mrs. Clinton by name, he referred to the controversies surrounding her use of a private email account while secretary of state and foreign donations to the Clinton Foundation, calling them “regrettable.”

Mr. Chafee’s long résumé belies what some have deemed a disappointing record. While he received praise for his overhaul of Rhode Island’s pension system as governor and for his education policy, he also oversaw a flagging state economy and was ensnared in several high-profile controversies, such as a debate over whether Christmas trees should be called holiday trees.