ANCHORAGE — Alaska’s ranked-choice voting system is emerging as a major concern for Republicans backing Sen. Dan Sullivan’s reelection, with a same-name challenger on the four-candidate general-election ballot and former Democratic Rep. Mary Peltola also in the race. Some Republican operatives fear Dan J. Sullivan Jr., a retired teacher, could take first-choice votes from the incumbent, Dan S. Sullivan. They are also concerned that supporters of the long-shot challenger may not rank the senator as an alternative. Under Alaska’s system, voters rank candidates in order of preference. If no candidate wins more than 50 percent of the initial vote, lower-ranked candidates are eliminated and ballots are redistributed according to voters’ next choices. Art Hackney, who leads a super PAC supporting the senator, said the challenger would receive “a percent of the vote” and that the impact could be significant. Two recent polls that modeled ranked-choice tabulations showed Peltola winning, adding to GOP worries about a closely watched Senate contest. Republicans have criticized ranked-choice voting for years. The Alaska race now puts the system’s practical effects—and the potential cost of a split vote—squarely before voters. President Donald Trump appeared with Sullivan at a campaign rally in Wasilla on Oct. 13, underscoring the national attention on the contest.