WASHINGTON — Nearly two dozen Democratic presidential candidates have crisscrossed the country for six months selling their vision for the United States. But, on the eve of the first debates in the campaign, a new poll from The Associated Press-NORC Center for Public Affairs Research shows most Democratic voters haven’t fully tuned in. Only 22 percent of Democrats registered to vote say they know a lot about the candidates’ positions, while 62 percent say they know a little.