Democratic Rep. Edward J. Markey remains the strong favorite to win the special election for Senate in Massachusetts, according to a new poll of registered voters released late Wednesday.

Markey led Rep. Stephen F. Lynch, his Democratic primary opponent, 50 percent to 21 percent, among polled potential Democratic voters, according to a UMass Lowell-Boston Herald survey. Twenty-three percent said they were were unsure about the primary.

Markey, a House veteran first elected in 1976, also led his all three potential GOP challengers by a comfortable margin in horserace matchups. His closest GOP competitor was former U.S. Attorney Michael Sullivan, who took 30 percent to Markey’s 48 percent. Sullivan is seen as the frontrunner in his race for the Republican nomination with state Rep. Dan Winslow and Gabriel Gomez, a private equity investor and former Navy SEAL.

Lynch also led all Republicans in horserace matchups.

But the poll also showed these special election candidates remain relatively unknown — unlike the recent high-profile 2012 race between now-Sen. Elizabeth Warren, D-Mass., and now-former Sen. Scott Brown, R-Mass.

Markey is the best known: 31 percent of those polled had a favorable opinion of him while 20 percent had an unfavorable opinion of him. Nineteen percent of respondents had never heard of Markey, while 31 percent said they had heard of him but were undecided.

The poll surveyed 589 Massachusetts registered voter from March 2-5 using live telephone interviewers with landlines and cellphones. The margin of error for the poll was 4 percentage points. Among the subset of 309 potential Democratic primary voters — registered Democrats and registered Independents who identify as Democrats — the margin of error was 5.6 points.