The Public Policy Polling survey shows Markey, a Democrat, a clear favorite to beat Rep. Stephen Lynch in a primary, while Brown might have an easier path running for governor.
Brown leads Markey by 3 percentage points, 48-45, at this point. But 69 percent of those currently undecided voted for Elizabeth Warren when she defeated Brown in November, while only 17 percent voted for Brown.
If that same breakdown occurs in the new race, Markey would lead Brown by a point.
Markey starts out with a 52-19 percent lead over Lynch. who has a net negative favorability rating with Democratic voters. Only 27 percent see him positively with 28 percent having a negative opinion.
Markey has better numbers -- 58 percent of Democrats have a favorable opinion of him to only 13 percent unfavorable.
The poll authors said the Senate race looks like about a 50-50 proposition for Brown now. Republican voters, by a 48-38 percent margin, would prefer Brown run for governor rather than the Senate.
Possible Democratic challenges have low name recognition.
PPP surveyed 763 Massachusetts voters and 404 Democratic primary voters by telephone Jan. 29-30. The margin of error is 3.6 percentage points for the overall sample and 4.9 percentage points for the Democratic portion.