Senator Bernie Sanders' supporters can be spotted wearing black shirts that read "Medicare for All." At house parties in Iowa, you'll sometimes see "Harris for President" hats. And on airplanes bound for Des Moines, you might see travelers with "Team Pete" pins and colorful "Mayor Pete" T-shirts.

Bernie Sanders campaign tee Sanders campaign

In a crowded race for the Democratic presidential nomination, merchandise can serve as a major source of marketing for presidential hopefuls. And in a primary cycle where, for the first time, making the debate stage also depends on meeting small-donor thresholds, having supporters spend $35 on a hat or just $4.99 on a set of stickers could go a long way.

Pete Buttigieg button Buttigieg campaign store

That's because when it comes to campaign merchandise, candidates do not have to separate purchases made from their online stores or at events from other donations when filing with the Federal Election Commission, so every purchase appears as another small-dollar donation.

"As small-dollar donations become more important to being able to show both local support but then also to technically get into the debate, we look at merch as a massive opportunity to raise funds outside of just begging for dollars via emails," said Victor Nguyen-Long, creative director for the progressive group ACRONYM.

"At the end of the day, everybody wants something in return for their money, right? And merch is a simple way to be able to get something tangible."

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With the many ways to capitalize on merchandise in mind, Campaigns are willing to dish out big bucks to build their brand and rake in more revenue.

In the first half of the year, FEC filings show, Mayor Pete Buttigieg's campaign doled out more than $1.2 million for gear; Bernie Sanders' campaign spent more than $900,000 for buttons, stickers, signs and T-shirts; Senator Elizabeth Warren's campaign paid more than $500,000 for merchandise orders; and former Vice President Joe Biden's campaign spent more than $150,000 on similar goods despite jumping in the race at the end of April.

So what are the candidates putting out? Clicking through hundreds of items online reveals campaigns are working to make sure they have something for everyone and keep their diehard supporters coming back for more. There's everything from t-shirts and yard signs to fanny packs and pet collars – even candidate branded calculators.

But what sells?

For entrepreneur Andrew Yang, merchandise is a major component of his fundraising strategy. His campaign announced in October that it raised $10 million in the third quarter of the year, with nearly a quarter of it came from merchandise sales. One of Yang's bestsellers is his "Make America Think Harder" (MATH) hat. When the campaign first launched the hats in March, they limited sales to just 500.

Andrew Yang campaign "MATH" hat Yang campaign store

The hats sold out in 22 minutes. Since then, Yang has sold 32,000 hats, raising about $1.2 million for the campaign or about 8% of all revenues as of mid-October.

According to the campaign, there have been several days of six-figure merchandise sales. When they announced a limited edition MATH hat with gold writing, the store sold 2,000 in just 36 hours, totaling $150,000 in revenue. Now, Yang even has a blue hat emoji in his Twitter handle, and supporters known as his #YangGang are also adding the emoji to their names on social media to signify their support.

"I took inspiration from street fashion, with limited editions, exclusivity, collectibles," said Yang's Director of Brand & Merchandise Andrew Frawley in a statement provided by the campaign. Other bestsellers for Yang have been a shot glasses, t-shirts with an image of Yang in high school, and "Math. Money. Marijuana" t-shirts, which came about after an analysis of Yang social media posts found tweets about legalizing marijuana performed well.

Meanwhile, it's the classics that have been providing consistent sales for Buttigieg's campaign, like their 2020 navy t-shirt or anything that says "Boot Edge Edge," the correct pronunciation of the South Bend mayor's name.

Limited edition gear has also sold well for Buttigieg, who's been rising in recent polls. The campaign says its one-off "Eco Tee" sold out in just six days. After a bee landed on Buttigieg's tie at a September campaign event in Iowa and refused to leave, they came up with a "Bee Tee" which was available for less than 18 hours. In the last four days of September, the campaign sold 2,700 "Rules of the Road" t-shirts, which hauled in more than $72,000 in sales.

Elizabeth Warren's campaign has been selling a variety of gear with its slogan "Dream Big Fight Hard," but they've also embraced a theme that's come up on the campaign trail. Warren has pushed a significant number of policy proposals, leading to the slogan "Warren has a plan for that." The phrase now adorns t-shirts, yard signs, totes, and beer koozies.

Elizabeth Warren can holders Elizabeth Warren campaign store

Another popular Warren motto comes from none other than Republican Senator Mitch McConnell. "She was warned. She was given an explanation. Nevertheless, she persisted," said the majority leader after a 2017 speech by Warren on the Senate floor was cut short. "She Persisted" now adorns t-shirts, bags, pencils, beer glasses (persist responsibly) and even "Purr-sist" cat collars on her campaign website.

Joe Biden's campaign is embracing the candidate's well-known name along with some of his signature looks. According to his campaign, the "Cup O'Joe" mug is its bestseller. His shop also pays homage to Biden's affection for aviator sunglasses with images of Biden in his signature shades. The image also embellishes t-shirts, tote bags, and buttons.

"From legacy items like the Cup O'Joe and staples like the Biden for President classic white tee to newer products like the patriotic Aviator Crewneck and the LGBTQ Pride collection - an issue important to the VP - we've seen a lot of excitement for all things Joe Biden," said Jamal Brown, the Biden campaign's national press secretary. Brown also said the campaign recently unveiled a new collection for the fall season, including shirts encouraging early voting.

Joe Biden "beat him like a drum" tee Joe Biden campaign store

And as Biden takes aim at President Trump, a new phrase is popping up on apparel. On the campaign trail, the former vice president frequently alleges that Mr. Trump solicited the Ukrainian government for help because he is scared Biden would "beat him like a drum" in a general election. His campaign website now sports a series of t-shirts covered in drums with the phrase.

Bernie Sanders, meanwhile, often likes to talk about how he changed the political conversation by bringing his left-wing policies into the mainstream. While his online store displays some traditional campaign gear with his name, it also has shirts, buttons, stickers, and mugs embracing signature policy proposals, like "Medicare for All" and "College for All." His website also brought back a popular phrase from his 2016 presidential bid with "Feel the Bern" fanny-packs and car magnets.

During the July Democratic debate, Sanders bragged that the Medicare for All proposal now embraced by many of his competitors was his idea. "I wrote the damn bill," he said to applause from supporters.

Before that debate even ended, his campaign emailed supporters soliciting donations in exchange for "I wrote the damn bill" stickers. Other candidates have also tried to monopolize on standout debate lines. Senator Cory Booker has "future president" merchandise, a reference to Biden calling him that at the July debate. Senator Kamala Harris sells t-shirts with her stand0out line from the June debate: "That little girl was me."

But when it comes to branding and merchandise, Mr. Trump has more than just an incumbent's advantage. When Mr. Trump took the stage at a recent rally in Dallas, he was greeted by a sea of supporters in red baseball caps. Those red MAGA hats, which are much more visible at events than t-shirts or pins might be, have been purchased en masse by Trump supporters since the 2016 Republican primaries.

FEC records show the Trump campaign in 2015 and 2016 spent more than $15 million alone at Ace Specialties for campaign merchandise including t-shirts, hats, stickers and yard signs. It spent more than $2.5 million at the company Cali-Fame, which manufactures hats.

As he mounts his re-election bid, Mr. Trump's campaign has spent more than $650,000 on merchandise in the first half of 2019 alone. Items range from hats and t-shirts to packs of recyclable straws embellished with Mr. Trump's name, a reference to the paper straws now used in several municipalities after plastic ones were banned.

Supporters participate at a campaign rally while awaiting the arrival of U.S. President Donald Trump and U.S. Senator Ted Cruz in Houston on October 22, 2018. Reuters

"President Trump is a master of communication and branding and his campaign merchandise is emblematic of that," Trump campaign communications director Tim Murtaugh told CBS News in a statement.

"Whether it's the over 1 million red MAGA hats sold, plastic straws, or fun one-off t-shirts, the campaign merchandise lets voters show their support of President Trump and his policies. It gives people a little ownership of the re-election campaign and gives them high quality merchandise in exchange for their donations."

As far as overall fundraising goes, Mr. Trump's campaign, along with the Republican National Committee, is currently leading individual Democrats in the race. But when it comes to small-dollar donations, Sanders surpassed the president in the third quarter. The Trump campaign said it received more than 1,056,000 individual donations during that period, while Sanders received 1,400,000 million individual donations.

It's unclear, however, what that translates to when it comes to campaign gear.

Eleanor Watson contributed to this report.