Part one of two. Click here to read part two.

*Correction appended.

Shortly before the primaries in March, voters in Amarillo, Odessa and Midland received a two-and-a-half page letter in the mail from Tim Dunn, a wealthy oil and gas developer who is generally averse to such publicity.

“Dear fellow West Texan,” Dunn began, before urging them not to cast ballots for incumbent state Sen. Kel Seliger, an Amarillo Republican who Dunn believed was not conservative enough.

“In every possible way, West Texas defines who I am as a conservative,” Dunn wrote. “And I am saddened to see West Texas values not represented in Austin.”

The letter was not from Empower Texans or any of the other scrappy yet influential political groups that Dunn has financed over the last decade. The return address was his home address.

“I am so concerned about this I am sending you this letter myself,” Dunn wrote. “No one is sponsoring it and I am not asking for money. I do ask you to hear me out.”

It was an unusually direct appeal from Dunn himself, who has not been the biggest spender in Texas politics but is widely seen as the most effective. Through a growing network of political groups that are often billed as grassroots, and frequently aligned with the Tea Party, Dunn has invested millions of dollars into moving the Texas Legislature further to the right.

While critics have decried his hardball approach to politics — the groups that he finances have made a mission out of knocking many long-serving Republican lawmakers out of office — Dunn has not backed down. In the last two years, he has increased his political spending while also expanding his focus to city council and school board races.

“I think his whole operation is designed to intimidate Republicans and clearly some of them are intimidated,” said Seliger, who won his primary with 53 percent of the vote.

Supporters of Dunn describe him as a thoughtful promoter of limited government who is at times misunderstood.

Graphic: Tim Dunn's Political Activity

“This narrative that he’s the really bad guy behind the curtain is really unfair in a lot of ways because what he’s trying to do is change the country,” said Brooke Rollins, president of the Texas Public Policy Foundation, an influential conservative research organization where Dunn is a vice chairman and donor. “While some people may disagree with the tactics, at the end of the day, he really is trying to do the right thing.”

Dunn, 58, grew up in the small town of Big Spring, northeast of Midland. He attended Texas Tech University, where he played lead guitar in a rock band. By the time he left with a bachelor’s degree in chemical engineering in 1978, Dunn was married with a young son. He scrapped plans to go to law school and began a career in the energy industry. In 1996, Dunn helped start the oil firm that would become CrownQuest Operating, where he still serves as chief executive.

Dunn’s work with groups advocating for limited government stretches back more than 20 years. Though he declined to comment for this article, previous public statements Dunn has made, as well as conversations with people who know him, reveal a portrait of a libertarian-leaning evangelical Christian who strongly believes that the success of small government policies in Texas helps to promote those concepts around the world.

“It is our job as Americans to do the hard work to perpetuate our market-based system of self-governance. … The rest of the world is depending on us,” Dunn wrote in 2012 on a conservative website.

Dunn’s political advocacy hit a turning point in 2006 when he became frustrated that Texas lawmakers were planning to adopt a new tax on businesses, an approach ultimately approved by Gov. Rick Perry. Dunn decided to launch a new organization focused on advocating for low taxes and spending restraint. To lead Empower Texans, he tapped Michael Quinn Sullivan, then a vice president at the Texas Public Policy Foundation who had previously served as press secretary to U.S. Rep. Ron Paul.

As soon as it launched, Capitol observers questioned whether Empower Texans was less about ideological purity and more about shoring up support for then-House Speaker Tom Craddick, a Midland Republican who lives less than a mile from Dunn. The organization’s leaders said at the time that they weren’t interested in internal House politics. Yet since San Antonio Republican Joe Straus replaced Craddick as speaker in 2009, the Empower Texans network has put substantial resources into unseating Straus and his top lieutenants in Republican primaries.

The Empower Texans network quickly tapped into the unrest of activists statewide who believed that conservative values were not being reflected in a state in which Republicans controlled every sector of government. Sullivan’s Fiscal Responsibility Index, which graded lawmakers on their conservatism based on their legislative votes, became a fixture at Tea Party gatherings.

State Rep. Jimmie Don Aycock, a Killeen Republican and chairman of the House Education Committee, received an A+ on the Index in 2009, followed by a C+ in 2011 and an F in 2013. He argued that the group cherry picks votes to create a false picture of the Legislature based on which races it is hoping to influence.

“I’ve sort of gotten to the point, if it looks or smells like Empower Texans, I probably look for reasons to vote against them,” Aycock said.

Dunn has stood by his organization’s scorecards, noting that Sullivan publicly announces throughout each legislative session which votes will be included and how they will be graded.

Republican incumbents who draw poor grades often find themselves facing primary challengers recruited by Empower Texans or its supporters. If a viable candidate emerges, Dunn’s network has been known to throw tens of thousands of dollars toward painting the incumbent as a liberal. In this year’s primaries, the group was credited with playing a role in the defeat of several incumbents, including state Sen. John Carona, a longtime Republican from Dallas.

Critics say the group’s funding contradicts its claim of being a grassroots organization. Dunn donated more than $2.5 million to the Empower Texans political action committee over the last year, 98 percent of its contributions over that period.

"It's not a movement,” said Bryan Eppstein, a Republican political consultant whose clients include several of Empower Texans’ targets. “It's a special interest agenda of one person."

Aside from the political action committee’s activities, an affiliated nonprofit “social welfare” group known as Texans for Fiscal Responsibility (TFR) is also run by Sullivan. Such “social welfare” groups are not required to report their donors unless electioneering is their “primary” function, and TFR does not. However the group’s recent spending on Republican primary candidates, which totaled more than $500,000, makes up a majority of the so-called dark money spent on political races during this election cycle, according to a report by Texans for Public Justice, a liberal watchdog group that tracks money in politics.

“His operation is running the biggest dark money pot we have so far in Texas,” said Andrew Wheat, research director for Texans for Public Justice.

Sullivan said most of his organization’s donors choose to give to Texans for Fiscal Responsibility, where their privacy is protected, over the Empower Texans political action committee.

"We have a very large and healthy donor base for TFR, for which we’re very grateful,” Sullivan wrote in an email. “We also appreciate that Tim has been willing to put his name and money out there on to the PAC list.” 

The Texas Ethics Commission is investigating complaints that a part of the Empower Texans network illegally solicited money for its PAC and that Sullivan should have to register as a lobbyist. Sullivan dismissed the investigations as politically motivated.

Dunn’s outsize influence in Republican primaries has turned him into an attack line. In some districts with contested races in the March primary, Eppstein sent out campaign mailers warning voters to be wary of efforts by groups funded by Dunn, who he described as “misleading Republican primary voters.”

Some critics have suggested that Dunn’s financial control extends to all of the groups with which he is affiliated, including the Texas Public Policy Foundation, where he spearheaded efforts on criminal justice reform that have since spread to other states. In 2012, the foundation’s donors were leaked to the public, revealing $4.7 million in contributions, including $43,000 from Dunn. Rollins said that figure was fairly in line with how much Dunn gives to the foundation annually.

“I think people think because of all his Empower Texans work that he’s the top giver to us as well,” Rollins said. “That couldn’t be farther from the truth.”

While Dunn has increased his recent spending on state races, he is also broadening his efforts. He has helped launch national organizations focused on combatting what he views as federal overreach, such as Obamacare.

He also gave $500,000 in 2012 to the Campaign for Primary Accountability, a bipartisan Super PAC that targeted unpopular congressional incumbents with underfunded primary challengers in several states. Leo Linbeck III, a Houston builder who has worked with Dunn on the Super PAC and other national nonprofits, said the two men share a belief that local governments can serve citizens better than Washington, D.C., as citizens are better positioned to hold officials accountable.

“I think we both saw the way the system was functioning and saw that the problem was more of a structural issue and the concentration of power,” Linbeck said.

Dunn is also expanding his focus to include nonpartisan local races, not just legislative and statewide ones.

Last year, Texans for Fiscal Responsibility opened a satellite office in North Texas and endorsed candidates in some city council and school district races. And both Empower Texans and Dunn played in the Midland mayor’s race, where they opposed councilman John James, arguing that his history of supporting Democrats made him the wrong choice to lead the West Texas city.

“I believe the Midland mayoral race is ground zero in the fight against big government liberalism and the effort to turn Texas blue,” Dunn wrote in a personal letter to voters.  

James, who lost to the Empower Texans-backed Jerry Morales, expressed disappointment that Dunn helped turn a nonpartisan race into a partisan one.

“The debate was clouded by a discussion about labels rather than a discussion about leaders,” James said. “Time will tell whether citizens across the state are well served by this tactic.”

*An earlier version of this story incorrectly described Leo Linbeck's background. 

Coming Wednesday: A look at Tim Dunn's investment in shaping Texas education policy. 

Disclosure: The Linbeck Family Charitable Trust is a major donor to The Texas Tribune. A complete list of Texas Tribune donors and sponsors can be viewed here.