WASHINGTON (AP) — In town halls, television interviews and social media posts, Democratic presidential candidates are touting their support for "Medicare-for-all," higher taxes on the wealthy and a war on climate change. But foreign policy, one of the chief responsibilities of a president, is largely taking a back seat on the campaign trail.

Former Vice President Joe Biden is seizing on that opening to position himself as the sole global policy expert in a crowded Democratic field if he decides to run for president.

In a series of speeches over the past month, Biden portrayed himself as an aut...