Former Republican Gov. Charlie Crist makes a campaign stop Sunday in North Miami, Fla. Crist, who switched parties in 2012, spent the weekend campaigning in the Democratic base of South Florida. (J. Pat Carter/AP)

On the wall of Charlie Crist’s new field office in the largely African American suburb of Miami Gardens, it is still possible to see traces of the previous tenant. Faintly visible through the fresh coat of white paint is a familiar blue O rising like the sun over red stripes.

The spectral presence of President Obama’s campaign logo was not all that evoked the not-so-

distant past as the likely Democratic nominee for governor dropped by Saturday morning.

Waiting for him was Lillie F. Ross, 70. Every Friday night during the 2012 campaign season, Ross gathered volunteers at her house to make phone calls on behalf of Obama. This Friday, she said, they plan to cook up some fish and start doing the same for Crist.

“We worked hard for President Obama, and we’ll work just as hard for Governor Crist — if not harder,” she said.

One of the biggest questions in politics this year and beyond is whether the spectacularly successful ground operation that Obama built for his two presidential campaigns will have lasting benefits for other Democrats down the road.

Florida Gov. Rick Scott talks to members of the media during a visit to a local business in Miami on Thursday. (Alan Diaz/AP)

The acid test may come this year in Florida, a state Obama won by less than a percentage point over Mitt Romney in 2012, where Crist is trying to unseat unpopular incumbent Rick Scott.

What may determine the outcome is how much of that old Obama campaign energy is transferrable to Crist, who was a Republican when he was elected governor in 2006, then switched to independent when he ran unsuccessfully for the U.S. Senate in 2010, before finally becoming a Democrat in December 2012.

Crist’s career as a Republican was effectively ended by a famous 2009 photo of him hugging Obama. Now he’s hoping that the president’s staunchest supporters will squeeze him back.

“The president’s campaigns, both of them, have really broken the mold on how you get out the vote,” Crist said in an interview. He recalled visiting Obama’s Tampa field office on Election Day 2012 and thinking: “My God, I’ve never seen this before. I can tell you, Republicans don’t know about this.”

Scott’s advisers scoff at Crist’s ground operation, which got started months after theirs did and has fewer than half as many field offices. One senior official of the governor’s reelection campaign dismissed it as a “tactical copy-and-paste” of Obama’s that “I don’t think the voters will buy.”

The closeness of the race and the depressing effect that its negative tone is likely to have on turnout has made it all the more important for both candidates to rev up their party bases.

“At the end, it’s going to be who has the best grass roots and can we get our message out,” Scott said in an interview. “We’ve got to have a better ground game than our opponents.”

While Crist is counting on the strength of a retrofitted Obama operation, Scott has had to build his practically from scratch. The campaign estimates that its volunteers have already knocked on 700,000 Florida doors — far surpassing the 200,000 that Romney’s campaign had reached by this point in the 2012 election.

Meanwhile, Democrats say the former Republican governor is making more of an effort in places such as vote-rich South Florida than their last gubernatorial nominee, Alex Sink, did.

“By far, by far — absolutely no doubt at all that he’s been engaged,” said Broward County Commissioner Dale V.C. Holness. “He’s everywhere.”

Holness, who is a political power in the Jamaican community, said that he is lending his real estate office to the Crist campaign as a call center, which is something of a tradition he started for Bill Clinton in 1992.

An early indication of how well Crist is connecting with his new party will come in Tuesday’s primary, where Crist is facing Nan Rich, a former Democratic leader in the state Senate. He is expected to win easily, but the margin of that victory, as well as voter turnout, will be watched closely as a sign of how well Crist is connecting with the Democratic base.

In an interview, Rich argued that voters in her party cannot trust a candidate who has pivoted “almost 180 degrees on issues that people would consider to be a core value of the Democratic Party.”

Republicans are eager to remind Democratic voters of that as well. Some of Crist’s former backers have organized a campaign placing what they say are 2 million calls to Democratic and independent households. Those who answer hear old recordings of Crist describing himself as a Reagan Republican who opposes abortion and same-sex marriage and supports publicly displaying the Ten Commandments. Democrats have filed a complaint with the Florida Elections Commission, citing a state law that prohibits phone calls that claim to represent someone without his or her approval.

The general election is expected to be the most expensive contest in the country, with Scott spending more than $100 million and Crist possibly half that much.

Outside groups are pouring in money on both sides.

Americans for Prosperity, funded by the billionaire Koch brothers, has been walking neighborhoods for Scott.

On the other side, mega-rich environmental activist Tom Steyer has started a Florida political committee and funded it with $750,000. He declines to say how much he will spend, but Democrats are hoping it could be as much as $10 million. His Nextgen Climate organization also has set up field offices across the state.

The battle has become as nasty as anything Florida has ever seen.

“I can’t remember a governor’s race that was so focused on the negatives of your opponent, rather than what you thought you could bring to the people if you were elected governor,” said Bob Graham, a Democrat who served two terms as Florida governor and three as a U.S. senator. “All of American politics, unfortunately, are getting more negative, but it’s also the personalities and the decisions the candidates have made to put all their energy and money toward attacking the other candidate.”

Thus far, Scott has dominated the airwaves, spending an estimated $20 million over the past six months. Crist and the Democratic Party did not get on the air until June. Independent fact checkers have given both sides generally negative reviews.

The deluge of ads has had an effect. Where polls a year ago had Crist holding a double-digit lead over Scott, more recent ones have the race in a statistical tie, with Scott holding a slight edge.

Voters’ views of both have become so soured that in late July, the political-handicapping Web site FiveThirtyEight wrote: “Democrat Charlie Crist and Republican incumbent Rick Scott are teetering on becoming the least-liked pair of candidates for any governor’s race in the past 10 years.”

But there is also plenty of contrast between the two, on issues and style.

Crist is the consummate politician. Working phones on Saturday afternoon with volunteers in Deerfield Beach, near Fort Lauderdale, he got a woman named Michelle on the line and quizzed her about her life: “You’re nine months pregnant and you’re due in a week? Well, then you should early-vote for sure!” When supporter Leanette Lockett, a ­45-year-old mother of three, recognized him in a parking lot in the Miami neighborhood of Liberty City, Crist insisted on putting one of his bumper stickers on her SUV himself.

Scott, four years after winning his first elective office, still has the stiff and sometimes awkward demeanor of the business executive he was. His biggest selling point is the economic recovery that is finally beginning to be felt in one of the states hardest hit by the 2008 global financial meltdown, and he takes credit for the 700,000 jobs the state has added since December 2010.

“We have had a big turnaround, and we can all feel it,” he told supporters Thursday at an Argentine steakhouse in Medley, a Miami-Dade County town where eight out of 10 residents count Spanish as their first language.

And then there is the surreal quality of a race in which a party-switching former Republican governor is pitted against the current Republican governor, who won his first election by running against the GOP establishment.

The scrambling of allegiances is dizzying: The unions that opposed Crist in his 2006 gubernatorial race are now showering his campaign with endorsements and money. Where Scott accused his 2010 primary opponent Bill McCollum of being “bought and paid for” by U.S. Sugar Corp., he has since accepted more than a half-million dollars from the company whose political influence he once denounced.

For longtime players in Florida politics, “It’s weird. It’s uncomfortable,” said Brian Ballard, who was once a leading fundraiser for Crist and now plays the same role for Scott — but still considers Crist a friend. “It’s Twilight Zone stuff.”

Alice Crites in Washington contributed to this report.