WASHINGTON — For four years, President Obama counted on fellow Democrats to rally to his side in a series of epic battles with Republicans over the direction of the country. But now, deep in his fifth year in office, Mr. Obama finds himself frustrated by members of his own party weary of his leadership and increasingly willing to defy him.

In recent weeks, disgruntled Democrats, particularly liberals, have bolted from the White House on issues like National Security Agency surveillance policies, a planned military strike on Syria and the potential choice of Lawrence H. Summers to lead the Federal Reserve. In private, they often sound exasperated describing Mr. Obama’s operation; in public, they are sometimes only a little more restrained.

They complain the White House has not consulted enough and failed to assert leadership. They say Mr. Obama has been too passive and ceded momentum to Republicans. Their grievances are sometimes contradictory; some grouse that he takes on causes he cannot win, while others say he does not fight hard enough for principled positions. The failure to enact tightened gun control laws and the Republican hold on immigration legislation have left liberals little to celebrate this year.

“If you read the papers, you almost think the Republicans are in control,” said Senator Bernard Sanders of Vermont, an independent who caucuses with Democrats and vigorously opposed Mr. Summers until he withdrew from consideration. “They’re constantly on the offensive. Democrats are on the defensive.”