Rep. Elijah E. Cummings would become the immediate front-runner if he entered the race to succeed retiring U.S. Sen. Barbara A. Mikulski, holding a double-digit lead over Democratic primary contenders Rep. Donna F. Edwards and Rep. Chris Van Hollen, according to a new Washington Post-University of Maryland poll.

Cummings (D-Md.) has the support of 33 percent of Democrats and Democratic-leaning independents, compared with 20 percent each for Edwards (D-Md.) and Van Hollen (D-Md.).

Should Cummings decide not to run, it will be good news for Edwards, the Post-U-Md. poll found. In a two-way race, Edwards leads Van Hollen, 38 percent to 28 percent.

But polling data also show the potential for shifts in the race. Six months before the April 26 primary, a quarter of the electorate is undecided or dissatisfied with their choices — and that number swells to one-third if Cummings is not in the race.

Reps. Donna F. Edwards (D-Md.) and Chris Van Hollen (D-Md.) are running in their party’s primary to replace U.S. Sen. Barbara A. Mikulski.

Cummings, a 10-term incumbent whose district includes portions of Baltimore and Baltimore and Howard counties, is the ranking Democrat on the House Select Committee on Benghazi. He has said he will announce his decision on whether to run after Democratic presidential candidate and former secretary of state Hillary Rodham Clinton testifies before the panel. Her appearance is scheduled for Thursday.

With registered Democrats outnumbering registered Republicans more than 2 to 1 in Maryland, whoever wins the primary will be favored to win the general election. Two Republicans are running so far: Chrysovalantis Kefalas, a speechwriter for the National Association of Manufacturers who worked in the administration of Gov. Robert L. Ehrlich Jr.; and Richard J. Douglas, an Iraq war veteran and former Justice Department official. Del. Kathy Szeliga (R-Baltimore County), the minority whip in the House of Delegates, has said she is determining her support.

Jonathan Scott, 63, a Baltimore construction supervisor, is one of the Democrats hoping for a chance to vote for Cummings. “He’s a man of the people,” Scott said. “I’ve voted for him at every opportunity. He’s in close contact with the citizens of Baltimore.”

Scott said he was especially impressed with Cummings’s steady presence on Baltimore streets in the spring during the unrest that followed the death of Freddie Gray while in police custody.

“He has always appeared to be a positive force in the city,” Scott said.

When Cummings’s supporters are asked about their second choice, about half say Edwards, and Van Hollen gets about a quarter.

Cynthia Brawner-Gaines, a retired Energy Department employee who, like Edwards, lives in Prince George’s County, said she has admired the four-term incumbent since her days as a neighborhood activist fighting the building of nearby National Harbor — and will vote for Edwards if Cummings does not enter the race.

“First of all, she’s personable,” said Brawner-Gaines, 63. “She started out in the grass roots in our area, and we’re pleased with her. She gets back with people regularly. She’s been doing that since we put her in [office] the first time.”

The Post-U-Md. poll shows significant challenges for Van Hollen, whether Cummings runs or not.

Despite a strong fundraising advantage over Edwards and endorsements from such high-profile black Democrats as state Sen. Joanne C. Benson, Montgomery County Executive Isiah Leggett and Prince George’s County Executive Rushern L. Baker III, Van Hollen sparks limited interest from African Americans, who make up roughly 40 percent of the Democratic electorate.

In a head-to-head contest with Edwards, Van Hollen trails 49 percent to 15 percent among African American Democrats. In a three-way contest, he draws 6 percent.

Van Hollen leads both Cummings and Edwards among white Democrats, but the split is much narrower: 30 percent for Van Hollen, 22 percent for Cummings and 17 percent for Edwards.

Both Cummings and Edwards are African American; Van Hollen is white. All three Democrats are reliably liberal; Van Hollen, a seven-term incumbent from Montgomery, has significant support from the party establishment, while Edwards has been embraced by national women’s groups.

But the survey also finds evidence that Van Hollen’s support may be more resilient in what’s expected to be a low-turnout contest. Van Hollen, a former state legislator, performs better among Democrats ages 65 and older and among those who say they voted in the 2014 midterm elections — groups that are more likely to cast ballots in a primary. In the state’s last primary election for Senate, in 2012, only 17 percent of registered Democrats voted.

Sally Walton, 57, a substitute teacher in Calvert County, said she’ll vote for Van Hollen because of his support for education funding.

“I just liked what I was seeing and hearing,” Walton said.

Earlier this year, Van Hollen called on Gov. Larry Hogan (R) to release school money set aside by the Maryland General Assembly. He also introduced legislation in the House that would provide annual increases in federal funding for special-education students.

The Post-U-Md. poll was conducted Oct. 8-11 among a random sample of 1,006 adult residents of Maryland, including landline and cellphone respondents. Full results have a margin of sampling error of plus or minus 3.5 percentage points. The error margin is plus or minus five percentage points among the sample of 550 Democrats and Democratic-leaning independents.