Troy Jackson is replacing Graham Platner, but Republicans are seeking to tie them together as a remade race kicks off with high stakes.

Democrats in Maine have reset a Senate race seen as critical to their party’s chances of retaking the chamber after an improvised scramble to swap out a damaged candidate, Graham Platner, for a new one, Troy Jackson, after a wild 17 days.

Now comes the tricky part.

Mr. Jackson, a former State Senate president, is trying to keep alive the grass-roots energy that Mr. Platner built over the past year while distancing himself from the former nominee’s personal history. With help from her allies, Senator Susan Collins, the five-term Republican he is trying to defeat, is seeking to tie the two Democrats together in ways the G.O.P. hopes will alienate swing voters.

A 100-day contest

This dynamic is beginning to shape a 100-day contest that will drench this largely rural state with money and place it at the epicenter of the national fight for Senate control. Democrats have turned to a familiar fixture of state government and a fifth-generation logger from Maine’s north woods to make their case against a far better-funded incumbent who has already defied national headwinds once in the Trump era.

The race will test whether Mr. Jackson can leave behind the baggage of a candidate, in Mr. Platner, who bowed out after a rape allegation that he denies, revelations about a tattoo that resembled a Nazi symbol and a trail of offensive online posts.

By replacing Mr. Platner with Mr. Jackson, Maine Democrats hope to make the race less about their candidate and more about painting Ms. Collins as a tool of the ultrawealthy and a rubber stamp for President Trump on the most pressing matters.