Donald Trump’s campaign drew contributions from more than 2.5 million supporters. (Jabin Botsford/The Washington Post)

When it launched in 2012, the GOP email list company Conservative Connector found steady business, collecting $500,000 from various congressional campaigns and several million as a subcontractor to Mitt Romney’s presidential campaign.

But the small Michigan-based firm’s revenue exploded after it was tapped last year to be the email list broker for the Republican National Committee, which was pouring money into beefing up its online fundraising operation for the 2016 presidential race.

By the end of last month, Conservative Connector had been paid more than $27 million to acquire and rent email lists by the RNC and a joint fundraising committee with Donald Trump’s campaign, Federal Election Commission reports show.

List brokers typically pass along a share of the money they are paid to other companies for access to their lists, pocketing a commission in the process, according to people familiar with the industry.

The gusher of cash that flowed to Conservative Connector illustrates the large sums at stake in the burgeoning email list rental industry, which is driving much of the online fundraising on the right.

In landing the RNC contract, Conservative Connector had a distinct advantage over its competitors: One of the heads of the firm, Ethan Eilon, provided consulting services to the party committee from mid-2015 to mid-2016, helping the RNC build its email fundraising, according to multiple people familiar with his role.

Reached by phone, Eilon referred a Washington Post reporter to the RNC press office. Another consultant tied to Conservative Connector, veteran Michigan strategist John Yob, declined to answer specific questions about the company’s contract with the RNC.

“We are proud of the hundreds of millions of dollars we have helped raise for Republicans across the country in recent years, including the RNC and President-elect,” Yob said in a statement.

Yob and Eilon, who were both involved in a contentious power struggle this year over which delegates would represent the U.S. Virgin Islands at the Republican National Convention, are not identified in Conservative Connector’s corporate filings. But several people with knowledge of the company said they are both involved in running the enterprise.

RNC officials, who spoke to The Post about the party’s online fundraising strategies in general, did not respond to later requests for comment about how the firm was selected.

Hungry for new supporters, candidates and party committees are lavishing money on vendors who promise access to potential donors. The treasure trove: fresh email addresses of people who have recently given to political campaigns or signed up on conservative news sites. Such data can be rented or sold without a donor’s knowledge.

“The profits are absolutely enormous,” said Steven Castleton, a New York consultant who is one of the pioneers in the business. “If you are a list owner with a good list, you stand to make a lot, a lot of money.”

That’s in large part because email is the engine that continues to drives the bulk of online fundraising. “It’s still the killer app,” said Gerrit Lansing, the RNC’s chief digital officer. The party sent out 2.5 billion emails to potential supporters this year, officials said, and almost two-thirds of the Trump campaign’s haul — which totaled $329 million by the end of November — came from donors who responded to contribution requests via email.

Digital strategist Patrick Ruffini said the RNC’s heavy investments in renting and acquiring new email addresses proved fortuitous because Trump had virtually no online fundraising operation when he secured the nomination. But in the long run, he warned, the GOP’s reliance on list rentals could burn out the donor pool, alienating people with a cascade of unsolicited appeals.

“Eventually, the money is going to dry up, because people are going to get tired of getting endless streams of messages from people they have no prior relationship with,” Ruffini said.

Kurt Luidhardt, an Indiana-based digital consultant who was one of the original founders of Conservative Connector, said the intense prospecting for new donors has opened the door to vendors hawking bad lists.

“You have to do a lot of vetting,” he said, calling the industry “enormous and a little out of control.”

Democratic digital strategists, for the most part, have taken a different tack, hunting for new donors through online ads. It’s more expensive than renting emails but guarantees that a candidate is appealing to a committed group of supporters, they said.

“The ideal is to build something organic where people understand what they’re opting into, and where you can have a relationship with them that can persist,” said Joe Rospars, who founded the firm Blue State Digital and was chief digital strategist for President Obama’s campaigns.

In this year’s presidential race, Hillary Clinton and the Democratic Party spent less than $3,000 on list rentals, according to federal filings. Her main challenger, Sen. Bernie Sanders of Vermont, also eschewed them.

“I don’t think people like spam, and I consider that spam,” said Keegan Goudiss, a partner at Revolution Messaging who served as Sanders’s director of digital advertising.

Clinton’s campaign ended up with more than 3.1 million individual donors, while 2.8 million people gave to Sanders, according to campaign officials.

Proponents of list rentals argue they are a more cost-efficient way to help a candidate rapidly tap into a small donor base.

“It actually can save a campaign money,” said Becki Donatelli, a top GOP digital strategist.

At the RNC, the party used rented email lists as one of the primary tools to expand its donor file this cycle. Together, the national party and one of its joint fundraising committees with the Trump campaign shelled out almost $32 million to 26 firms to rent email lists, FEC reports show. Another $8.7 million went to run online ads, many aimed at finding new donors.

“Email list rentals can be incredibly effective — mostly because you’re contacting known donors and it can scale very big, very fast,” Lansing said. “If you can spend a dollar on any channel — whether it’s ads or email — and get more than a dollar back, you should never stop doing that. Ever.”

Conservative Connector, which rents email lists that it owns and serves as a broker for other list vendors, was in hot demand this cycle. In the GOP primaries, it was hired directly by former Florida governor Jeb Bush’s campaign, a pro-Bush super PAC, former ­Hewlett-Packard chief executive Carly Fiorina’s campaign, a pro-Fiorina super PAC and New Jersey Gov. Chris Christie’s campaign, finance reports show.

The RNC first paid Conservative Connector in July 2015 — a month after Eilon began getting paid by the party as a fundraising consultant through his other firm, Vertical Strategies, filings show. Several people familiar with his role said he helped build and organize the RNC’s email fundraising operation.

Payments to Conservative Connector ballooned as Election Day neared. Over two days in late October, the company collected $6.6 million from the Trump Make America Great Again Committee, according to campaign finance reports. Another payment for $4.7 million came Nov. 10, two days after the election.

RNC officials and Yob declined to say how much commission the firm received. Typically, email brokers charge between 15 and 25 percent, but the fee for such a large account may have been closer to 10 percent, according to people familiar with the industry.

Conservative Connector’s website and public filings give little insight into the ownership or corporate structure of the firm.

Luidhardt said he and his wife, who co-founded the firm in February 2012, sold their share in early 2015. He declined to name who the other owners were at the time.

That December, a lawyer filed new incorporation papers for Conservative Connector in Michigan, listing as its agent David Dishaw, who is also a registered agent for Vertical Strategies. On corporate disclosures, Conservative Connector lists the same Grand Rapids address as several companies run by Yob and Eilon.

Yob is a well-known strategist in GOP circles. He served as deputy political director for Sen. John McCain’s 2008 White House campaign, ran Michigan Gov. Rick Snyder’s successful campaigns in 2010 and 2014 and was national political director for Sen. Rand Paul’s presidential bid this cycle.

He and Eilon have worked together on a number of campaigns, including former representative Pete Hoekstra’s 2012 Senate bid in Michigan and Ben Sasse’s 2014 Senate run in Nebraska.

In late 2015, both Yob and Eilon relocated with their families to the U.S. Virgin Islands, voter registration records show. There, Yob used a limited-liability company to purchase a $4 million villa on St. John, according to property records. The expansive estate features an indoor and outdoor pool, a waterfall and sunset views of the St. Thomas and Cruz Bay harbors.

The two men drew headlines this spring when they and their wives sought to run as the territory’s delegates to the Republican National Convention, setting off an ugly fight within the local party.

Yob’s faction ultimately prevailed this summer when the party’s contests committee determined that his slate should be seated. His victory was touted in blog post on Conservative Intel, a website run by Conservative Connector that collects emails through a weekly newsletter offered to subscribers and a political merchandise store. The website also promoted a book called “Chaos” that Yob wrote about how a contested convention would work.

By the end of the November, Conservative Connector had collected more than $30 million from its political clients for the cycle, FEC filings show.

The RNC, which headed into the general election with a list of less than 5 million supporters, amassed a pool of more than 10 million email addresses with the Trump campaign — including those of more than 2.5 million individual donors, officials said. The vast majority were obtained through the joint fundraising committee, which directs 20 percent of its proceeds to the national party.

After Trump’s victory, that list is already paying dividends. After Nov. 8, donors gave millions more to his campaign and the party as they were peppered with emails offering merchandise such as “Make America Great Again” Christmas tree ornaments and an official “Trump Presidential Cap.”

The Trump-RNC email list could be a potent tool for a group being formed to support the new president’s agenda that may be run by one of his top advisers, Kellyanne Conway.

“Whatever the Trump team wants to do in terms of legislation, helping candidates, raising money — the sky is the limit,” Lansing said. “There’s a ton of potential with this size of a list.”

Correction: Ethan Eilon’s name was misspelled in an earlier version of this story. It has been corrected.

Alice Crites and Anu Narayanswamy contributed to this report.