By Jen Christensen, CNN

Charlotte, North Carolina (CNN) - It’s 10 p.m. in Charlotte’s trendy NoDa district, and a handful of women and one man have hunkered down over French press coffee and caramel pastries that are so amazing the cafe’s owners were once invited to the White House.

These customers have gathered at Amelie’s, a 24-hour French bakery, with a serious mission. They want to end human trafficking.

As volunteers for Compassion to Act, a faith-based nonprofit, they meet regularly to discuss how to rescue and restore the lives of human trafficking victims. But with the Democratic National Convention gathered just down the road this week, they and other groups have stepped up their efforts.

They anticipate a greater need for their help.

More than 500 miles away in Tampa, a Christian ministry called Created also beefed up its volunteer patrols during last week’s Republican National Convention.

The group, which focuses on “vulnerable women involved in the sex industry,” immediately saw evidence the convention was in town. On their rounds, volunteers encountered a woman who looked out of place in too-nice clothes on Nebraska Avenue, a Tampa street notorious for its sex trade.

As they handed the woman a purple purse filled with supplies – a condom, some lip gloss, a stack of chocolate chip cookies and a phone number for help - she admitted she had taken a bus down from St. Louis when she heard there’d be extra work.

Groups like Created and Compassion to Act say they’ve seen this happen countless times. Whenever a large event comes to town - be it a Super Bowl, a NASCAR race, even a religious event such as Promise Keepers - the groups see a lot more men out cruising for sex on their city streets. The underground sex industry meets that demand - whether the women involved want to or not.

“People find a way to exploit a woman or girl’s vulnerability, whatever that may be, and they find a way to take advantage of that broken spirit,” says Kim, one of the Compassion to Act volunteers. She asked that her last name not be used because of the sensitive nature of her work.

“They force them into this life promising drugs, or nice clothes, or even something as simple as an initial boost to the girl’s self-esteem. She is manipulated to do this, whether the girl is aware or not. We haven’t met a single person who says they want to be doing this.”

A 2011 Baylor University study, “Men in Transit and Prostitution: Using Political Conventions as a Natural Experiment,” examined the number of online ads for commercial sex during the 2008 conventions. It found the number of ads in the host cities rose by 30%.

The anti-trafficking group Shared Hope International says the problem gets worse during large events but is always present. The group assesses state laws aimed at trafficking and issues report cards: Florida earned a C; North Carolina got a D.

To fill in the gaps left by the states, volunteer groups work to raise awareness and conduct street outreach to help the women involved.

To catch the attention of the country’s leaders during this year’s political gatherings, Shared Hope rented mobile billboards and drove them around the convention sites. Volunteers hope delegates will notice their campaign’s startling image: A man with a beer belly opens his suit jacket and reveals RNC and DNC tie tacks on his tie. The words next to him read, “This man wants to rent your daughter.”

“Our message is not to accuse America’s political leaders of engaging in the commercial sex industry,” Shared Hope founder and president Linda Smith said in a statement. “On the contrary, we are asking them to no longer be disengaged on the issue.”

“As the leaders of our nation, we need lawmakers to stand against the driving force of the illegal commercial sex industry that claims thousands of American children each year,” said Smith, a former congresswoman. “That driving force is demand.”

Experts estimate that 100,000 children a year are exploited in the U.S. commercial sex industry, according to Shared Hope. The average age of a child first exploited through prostitution is 12, the group says.

To counter the demand, Compassion to Act has beefed up its boots on the ground in Charlotte.

After planning their strategy over coffee and pastries, Kim and fellow volunteer Aimee hit the city’s strip clubs, where they’ve built fragile friendships with the dancers.

On their weekly visits, the volunteers are invited into some of the dancers’ crowded dressing rooms. Kim and Aimee offer to help with whatever the dancers may need – food, a ride to a doctor’s office, even school supplies for their kids.

“We listen. We make sure they know they are loved,” Kim says. “And when they want out, we have the resources in the community to help them do that.”

Back at the bakery, group co-founder Debbie Hancock sits with her iPad and scours the property ads looking for suspicious rentals. She discovers a listing for a mansion renting for the week of the DNC for 16 times its usual price. “That doesn’t look right,” she says.

Upon such discoveries, Hancock and Ish Payne - a retired cop who also helped found Compassion to Act and has written a book on the subject - will either alert local police or conduct their own surveillance at a safe distance, watching to see what kind of traffic goes in or out.

“If there is a lot of traffic right before work or on people’s lunch breaks, cars with out of state tags – these are some of the red flags,” Aimee explains. Sure it could be a legitimate rental - a political big wig in town for the convention, for instance - but it could be something different. “Temporary brothels pop up even in the nicest neighborhoods, and in the Charlotte area authorities do want to know.”

Some cities are better than others at cracking down on the illegal sex trade during these large events.

Abby Kuzma, a lawyer with the Indiana attorney general’s office, says Dallas came under a lot of intense criticism for an uptick in the sex trade when it hosted the 2011 Super Bowl. After her office learned Indianapolis would be the next host city, it put together a coalition of city leaders and about 60 nonprofits and immediately reached out to the host committee.

“We first told them it is not the Super Bowl per se that attracts human trafficking,” Kuzma said. “It’s any large event where there are a lot of men - like you have with the political conventions now - who are frankly looking for a good time.”

The Indianapolis coalition trained hotel staffs, local volunteers and cab drivers, teaching them what to watch for and who to call for help. Another nonprofit, the S.O.A.P. Project, stuck a national hotline number on the bars of soap it handed out to hotels so victims could call for help from the privacy of a bathroom.

This proactive approach was so successful, Kuzma says, that they saw immediate results. The number of online sex ads went down. And men seeking illegal sex warned each other online to stay away from Indianapolis.

“The demand went down because the men were feeling Indy had a negative atmosphere for the sex trade,” Kuzma said.

It’s unclear what strategy Charlotte has in place. City officials told CNN they weren’t commenting on this type of policing for security reasons. Compassion to Act, which says it has good relationships with law enforcement, was told there is a plan in place for the DNC but couldn’t be given details, again for security reasons.

In Tampa, Created heard something similar, although Tampa Mayor Bob Buckhorn told CNN police would be too busy during the conventions to raid strip clubs.

In both cities, the groups decided to step up their presence. Indiana’s Kuzma says that’s a good idea.

“Certainly the nonprofits help a lot,” she said. “Law enforcement is overwhelmed dealing with crowd control and other security issues. Throw in human trafficking, and it can be a lot to handle.”

On Monday night, while Compassion to Act volunteers talk about how they can do even more during this week’s convention, Debbie receives an urgent text from a minister. He’s been working with a trafficking victim and wants to know if the group can help. Having prepared for this week, the answer is yes – and they begin arranging emergency shelter for the woman.

“Sometimes this problem with human trafficking feels too big and overwhelming to stop,” says Kate Stahlman, another Compassion to Act co-founder. “But then God always finds a way to let us know – even in small ways – that what we do matters.”