Jim Webb could be Hillary Clinton’s worst nightmare.

The former one-term Virginia senator and Vietnam War veteran is making sounds about running for president as a Democrat. He was in Iowa last month; a New Hampshire trip may be in the offing, and he’s giving a major speech at the National Press Club in two weeks.

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 fundraising network.

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

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

As secretary of state, Clinton was the chief advocate in the Barack Obama administration for intervening against Moammar Gaddafi. When the Libyan dictator was toppled and killed in 2011, she thought it would be her signature foreign-policy achievement.

Webb, then a senator, adamantly opposed this venture. The U.S. has since withdrawn its personnel from Libya, and radical jihadists now occupy a compound belonging to the U.S. embassy.

Clinton recently said she disagreed with Obama’s decision not to intervene in the Syrian civil war. Webb warns that the Syrian opposition includes not only elements friendly to the U.S., but also the radical Islamic State forces that have wreaked mayhem there and in Iraq, murdering thousands and beheading two American journalists. Syria, he has warned, is “Lebanon on steroids.”

Clinton has close ties to Wall Street, a source of campaign funds for her and the Clinton Foundation. Since leaving office, she has received large speaking fees from hedge funds, private-equity companies and big banks such as Goldman Sachs Group Inc.

He pushed a measure to slap a special tax on big bonuses paid out by Wall Street companies that received government assistance during the financial crisis. When it failed, he complained that Democrats, beholden to Wall Street, killed it.

If Webb decides to run — fearlessness and unpredictability are his trademarks — there’s plenty of ammunition against him. He’s against gun control, and he has made comments that angered feminists, many of whom consider Clinton a cause as well as a candidate, and environmentalists. He also has been involved in numerous personal controversies.

In a recent Virginia Senate debate, Republican Ed Gillespie sought to paint the moderate Democratic incumbent, Mark Warner, as too left, citing occasions when he didn’t join Webb in voting along a more conservative line.

The maverick lawmaker had a few notable successes, passing a major veterans’ education bill, putting criminal justice reform on the agenda, and calling for a pivot to Asia before Obama was elected. He has criticized executive overreach by both Bush and Obama.

A decorated war hero — he received the Navy Cross for “extraordinary heroism” — and author of nine books, he would run principally on the issues most likely to cut Clinton: opposition to an interventionist-centered foreign policy and softness toward Wall Street. He would bring more authenticity to these two issues than any other would-be Clinton challenger. In Iowa, he made no secret of his criticism of Clinton’s tenure at State.

Clintonites will dismiss the Webb threat by pointing to his political weaknesses. But here’s a safe bet: They will closely monitor his Sept. 23 Press Club speech.

Albert R. Hunt is a Bloomberg View columnist. He was formerly the executive editor of Bloomberg News, directing coverage of the Washington bureau.