Democrats appear to be holding off Republican attempts to flip Senate seats in Michigan and New Hampshire but are struggling in their efforts to oust a Republican in Maine, according to New York Times/Siena University polls of three battlegrounds. In Michigan, the party’s nominee, Dr. Abdul El-Sayed, leads his Republican opponent, former Representative Mike Rogers, by five percentage points. In New Hampshire, Representative Chris Pappas is running five points ahead of former Senator John E. Sununu.

But in Maine, Senator Susan Collins holds an edge of three points over Troy Jackson, her Democratic challenger. Her showing, which comes after a sudden switch of the Democratic nominee, could fuel Democratic worries that they will again lose to Ms. Collins. The results underscore the uncertainty both parties face in tight Senate contests. To win control, Democrats must flip four Republican-held seats and keep all of their own.

The poll found that independent voters were deeply divided. In Michigan and Maine, independents were nearly evenly split. In New Hampshire, they leaned toward Mr. Pappas by 10 points. Democratic candidates are lifted by broad dislike of President Trump, with nearly two-thirds of voters describing him as focused on the wrong issues. Yet Republican nominees outperformed Trump’s 35 percent approval rating, sometimes by double digits. While a majority view the G.O.P. as too far right, most do not see their local candidates as too extreme.