BOSTON — Democratic Sen. Ed Markey holds a “significant” lead over his primary rivals, according to a new poll, which suggests challengers will face a difficult fight to unseat the three-term incumbent.

The University of Massachusetts at Amherst/WCVB TV poll , released Sunday, showed Markey with a 20-point lead over Congressman Seth Moulton and former teacher Alex Rikleen in a Democratic primary match-up. The poll of 800 likely voters found 51% supported Markey, compared to 28% for Moulton and 6% for Rikleen. About 13% said they were undecided.

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