In the WBUR/MassINC. poll, Markey takes 35 percent support to Lynch's 24 percent support among likely Democratic primary voters, a slight decline in support for both of them since February, when Markey took 38 percent support and Lynch took 31 percent support.

The rise in voters who aren't sure which candidate they'd support could indicate they're beginning to tune in to the race a month out from primary day, and haven't yet made up their minds.

Markey and Lynch's first messaging attempts seem to be hitting home, however. Lynch has focused on his support for veterans on the trail, and voters who care most about the military prefer him, while Markey touted his work on gun control in his first ad and takes more voters who care about that issue.

Neither candidate has yet launched any attacks on the other, but Markey heads into the final month of campaigning seen less favorably than Lynch: 32 percent of respondents report having a favorable impression of him, while 23 percent have an unfavorable impression. Thirty-seven percent have a favorable opinion of Lynch, while 12 percent have an unfavorable impression.

For both candidates, however, about 30 percent of respondents say they've heard of them but are undecided. That portion of undecideds indicates an opportunity for each candidate to work to define the other, and potentially raise his opponent's negatives.

That tactic may have backfired for former Sen. Scott Brown (R-Mass.) in his 2012 race with Sen. Elizabeth WarrenElizabeth Ann WarrenJuan Williams: Trump gives life to the left Elizabeth Warren urges grads to fight for 'what is decent' in current political climate Tomi Lahren responds to genealogist's investigation of her family: 'She failed miserably' MORE (D-Mass.). His attempts to portray her as an elite, out-of-touch academic may have ultimately lessened his popularity among voters.

On the Republican side, former George W. Bush administration official Michael Sullivan takes a strong lead with 28 percent support to state Rep. Dan Winslow's 10 percent support and local businessman Gabriel Gomez's 8 percent support. Forty-six percent of likely Republican primary voters say they don't know who they'll vote for.

If Sullivan does win the GOP primary, however, Lynch fares better against him than Markey, taking a 28-percentage-point lead over Winslow to Markey's 17-percentage-point lead.

While both Democratic contenders lead every possible GOP nominee by double digits, Lynch posts a larger lead against every one.

The survey was conducted among 610 likely voters from March 19-12, and has a margin of error of 4.1 percentage points.