Hillary Clinton's struggle in Iowa to fend off underdog Bernie Sanders, a self-described democratic socialist, reignited questions about her ability to close the deal with Democratic voters and turned up the pressure on her high-profile White House campaign.

The Democratic presidential front-runner, whose campaign ran off the rails in Iowa in 2008 against Barack Obama, was dealt another setback on Monday in the Midwestern state that begins the 2016 race for the presidency.

The former secretary of state, Clinton, 68, was pushed to a virtual tie with Sanders, a 74-year-old U.S. senator from Vermont.