WASHINGTON — Jim Webb could be Hillary Rodham Clinton’s worst nightmare.

Mr. Webb, a Vietnam War veteran and Democrat who served one term in the Senate representing Virginia, is making noise about running for president. He was in Iowa last month; a New Hampshire trip may be in the offing, and he is giving a major speech at the National Press Club in Washington on Sept. 23.

He seems an improbable candidate. He has taken illiberal positions, was President Ronald Reagan’s Navy secretary, has few relationships within the Democratic Party, and has no serious fund-raising network.

What he does possess is a long-held and forceful opposition to American interventions in Iraq and Libya, and potentially Syria, as well as solid anti-Wall Street credentials. In Democratic primaries, these may be Mrs. Clinton’s greatest impediments to rallying a hard-core activist base.

In 2002, Mr. Webb warned of the perils of invading and occupying Iraq; he has been proved right by the violence and sectarian strife of the post-Saddam Hussein era. As a senator, Mrs. Clinton voted for the war and supported it for years. She recently acknowledged that she had been wrong.