Another poll of likely Democratic presidential primary voters shows Vermont senator Bernie Sanders closing a considerable gap with Hillary Clinton, while Vice President Joe Biden looks like he could be a spoiler if he decides to run for the nomination. The latest Monmouth University poll finds Clinton with a slim majority of support at 51 percent, while Sanders and Biden (who is not yet running) receiving 17 percent and 13 percent support, respectively.

For Sanders, a democratic socialist and independent running for the Democratic nomination, that 17 percent is up 10 percentage points since April. Meanwhile, Biden's 13 percent is steady for Monmouth's polling over the last few months. The poll also asked if they would consider supporting Biden if he got in the race. Combining those who already support Biden with those who say they are "very likely" and "somewhat likely" to consider supporting him gives Biden a total of 56 percent who may or already do support him for president.

Furthermore, a majority of Democrats and Democratic-leaning voters—53 percent—say "it would be better" if Clinton faced an active primary challenge than if Democrats lined up behind the former secretary of state.

According to the Real Clear Politics average of polls, Clinton remains far ahead of Sanders, Biden, and other Democratic challengers, though her support has been steadily declining since the beginning of the year.