A new Bloomberg/Des Moines Register Iowa poll shows the Vermont senator trailing Clinton at 39 percent, a 2 percent bump from October's survey. Clinton has stretched her lead from 42 percent in October to 48 percent on Monday.
Trailing far behind is former Maryland Gov. Martin O'Malley at 4 percent.
Bloomberg reports Clinton garnered greater support from seniors, women and the wealthy, and was also helped by terrorism concerns. However, Sanders may be able to close the gap if he appeals to first-time caucus-goers and millennials.
A poll released by the Harvard University Institute of Politics last week found Sanders to have a greater appeal among millennials. Forty-one percent of those polled would choose Sanders for president, compared to 35 percent who would choose Clinton.
Bloomberg and the Des Moines Register surveyed 404 Democratic likely caucus-goers between Dec. 7-10 with a margin of error of +/- 4.9.