Mayoral candidate Muriel Bowser greets voters that she had transported Wednesday to an early-voting site in the District. (Jahi Chikwendiu/The Washington Post)

Mayoral candidate Muriel Bowser is surging in polls, but her support is squishy: Those turning to her are less likely to vote, and they still might change their minds.

Incumbent Vincent C. Gray enjoys a rock-solid base of die-hard supporters, but there are far fewer of them than there used to be.

Both circumstances point to the urgency of turnout for both front-runners in Tuesday’s Democratic primary.

Bowser is working feverishly to identify the supporters who, according to a new Washington Post poll, are flocking to her. She must persuade them to stick with their choice — and make sure they actually vote.

Since January, Bowser’s support has more than doubled, and she is now in a statistical dead heat with Gray and far ahead of six other contenders, according to the poll. She has drawn in previously undecided voters and peeled away support from the competition. She has gained heavily among white voters and is second only to Gray among black voters.

But with the cavalcade of challengers differing little on ideological or policy grounds, Bowser’s emergence may be grounded chiefly in her emerging role as the alternative to Gray. That may help explain why only 44 percent of those who support her say their choice is definite. For Gray, that amount is closer to two-thirds.

Gray’s ability to attract additional supporters has stalled, according to the poll. His bedrock from 2010 — African Americans east of the Anacostia River — is dramatically thinner. As a result, he has assembled an army of professional campaigners who are using time-tested techniques — door knocks and phone calls — to turn out every last person still rooting for him who helped deliver his overwhelming victory four years ago.

Just over half of registered African Americans say they are certain to vote on Tuesday, down 16 percentage points compared with four years ago. In wards 7 and 8, the trend is even more pronounced: The percentage of voters saying they are certain to turn out has dropped from 67 percent to 48 percent.

Citywide, turnout for the city’s earliest-ever mayoral election is expected to be low. Just 54 percent of registered voters say they are “absolutely certain” to vote, down from 65 percent three weeks before the 2010 contest. And that number may be optimistic: 40 percent actually turned out in 2010.

To overcome their soft spots with voters and the uncertainty surrounding voter interest, Gray and Bowser are following divergent game plans. Both are scrambling to finish a months-long process to identify likely voters — and transition to contacting each one to urge him or her to vote.

The van with the green “Muriel for Mayor” sign in the dashboard pulls up, and the driver doesn’t waste time finding a place to park. Campaign staffers help a handful of the oldest residents at a Southeast apartment tower climb out and get in line.

Vote. Reload the van. Repeat.

Five miles away in Northeast, a rented school bus with blue “Vince GRAY for Mayor” signs taped on the windows rolls up to the polls after an hour of picking up the seniors and residents of a public housing complex. Another busload to Bowser’s many vanfuls.

Both campaigns — but especially Gray’s — wish there were still more voters to be had like Stafford I. Pemberton. The 93-year-old Southeast resident is “absolutely” sure to vote, he said, with the help of a ride to the polls from his brother.

In the final days, he remains open to being swayed by phone calls and door-knocking.

“I’m hearing more about Bow­ser, but I just got a call from the mayor’s campaign. They were convincing,” said the retired Baltimore teacher. “I’m thinking I may vote for the mayor — I don’t see anything bad that he’s done since he was elected.”

It was what Gray’s campaign associates did four years ago at this very point to help get voters to the polls that invites inevitable comparisons — and that has left the incumbent vulnerable to defeat in a city where most think things are on the right track.

Five people connected to Gray’s 2010 campaign have pleaded guilty in connection with more than $600,000 in off-the-books spending to mobilize supporters and blanket the city in Gray’s campaign colors.

Whether Gray can be as effective again without the illicit spending remains unknown.

Chuck Thies, the mayor’s campaign manager, says the answer is emphatically “yes” because he has built a professional campaign that would never embarrass the mayor in the same way.

With Gray’s past campaign apparatus decimated by the federal investigation and the mayor’s last-minute decision to enter the race in December, Thies had to assemble the bulk of a citywide campaign by writing checks. He has spent more than $300,000 on consultants and is likely on his way to well north of $500,000, he said.

One firm was tasked with running computer modeling of everything from residents’ magazine subscriptions to their salaries to identify potential new voters; another filled out call centers across three states and began working through Gray’s list of supporters from four years ago.

With only weeks to organize a citywide canvassing operation, paid consultants were brought in to manage each team of door-knockers. They followed mailings, and potential voters got callbacks, then more-probing ones. In all, the campaign has been paying for 7,000 to 8,000 calls a night to city residents, he said.

Thies said the campaign has e-mail addresses for 743 potential volunteers, but the emotionless analysis that the paid outsiders have brought to the operation has given the campaign a clearer sense than anyone of Gray’s strengths and weaknesses and which voters he can turn to in the final days for votes.

“Four years ago, you could have pretty much drawn a line down the middle of the city and said, ‘Get everyone on this side to vote, and everyone on that side not to,’ ” Thies said. “In this campaign, there is no line. . . . We’re talking about a sophisticated, surgical strike. We cannot carpet-bomb our way to victory.”

Bowser is hoping that her surge continues to snowball into a clear lead. Her campaign has changed its script for phone-bank calls to reflect the pitch: “Muriel Bowser is the only candidate in the race for mayor with the organization and vision to win on April 1st,” began each of more than 2,000 calls placed Tuesday by volunteers at her Georgia Avenue campaign headquarters.

Bowser’s e-mail list of volunteers is near 1,500. A dozen or more walk into the converted convenience store daily. Some pop in just for a button but get talked into sitting down for an hour or two to make calls.

Most important, on a laptop in a backpack that never leaves the side of campaign manager Bo Shuff, there’s “The List.”

Bowser was Adrian M. Fenty’s Ward 4 campaign coordinator during his first run for mayor. Her campaign chairman and campaign strategist were Fenty’s, too.

During that successful run, the team developed a target number of voters for each of the city’s precincts. They have done so this time, too; the highly guarded “List” includes the number believed to be needed to win in each ward, precinct and neighborhood.

“We feel good about the work that we have done,” Shuff said. “The question is, will we turn out the number we thought we would, and is that enough, because you don't know what the other side is doing.”

The increasingly important pre-game for the two sides is already underway: early voting. The Post poll found that 21 percent of likely voters, or twice as many in 2010, plan to vote early or absentee.

Thies, from the Gray campaign, took voters by the busload to early voting locations — from Chinatown on Monday and Fort Lincoln on Tuesday.

But on a blustery Wednesday morning, few were taking. At King Greenleaf Recreation Center, the bus arrived empty after four stops at senior citizen centers.

Bowser’s vans, headed to the Hillcrest Recreation Center, in Gray’s home turf, were full with 30 voters. The candidate herself showed up to say thanks. The Rev. R. Joyce Scott embraced Bowser.

“Can you call five more, 10 more and have them come out tomorrow?” Bowser asked. Scott nodded yes. “Good, we’ll send another van,” Bowser said.