WASHINGTON — Congressional Democrats, torn over involving the United States in another unpredictable Middle East war, are emerging as a major barrier to President Obama’s plan to strike Syria.

Many of the president’s core supporters, especially African-Americans and members of the Democratic Party’s liberal wing who voted repeatedly against the wars in Iraq and Afghanistan, are expressing the deepest reservations. With rank-and-file House Republicans showing little inclination to back Mr. Obama on an issue on which he has staked his political credibility, scores of Democratic votes will be needed if a resolution authorizing force against Syria is to pass the House.

Democrats say they are being confronted with a difficult choice: Go against the wishes of a president who is popular and well respected in their caucus, or defy voters back home who are overwhelmingly opposed to another United States military intervention overseas.

In the first sign of how splits within the party will loom large over the Syria debate, two Democrats voted no and a third voted present on Wednesday when a divided Senate committee approved a use-of-force resolution with senators from both parties crossing over.