Janet Mills stopped short on Tuesday of endorsing her party’s nominee, Troy Jackson, for U.S. Senate. Mills dropped out of the Democratic Senate race in April. A New York Times/Portland Press Herald/Siena poll conducted in June, when Graham Platner was the Democratic nominee, showed Mills’ favorability at minus-8 percentage points. But a separate survey by the University of New Hampshire Survey Center earlier this month showed Jackson’s favorability above water — at plus-5 points. (Mills endorsed Hannah Pingree, the eventual Democratic nominee, in that race.)