It’s a case of poacher turned game-keeper. US Democrats, reeling from losses in the midterm elections, are turning to a former Republican media hitman to boost their chances of taking the White House in 2016.

David Brock is the name; his trademark, a silver pompadour and Trotsky-style wire-rimmed glasses; his political ethos, to beat Republicans by using an apparatus of quick-response law, ethics groups and journalism groups, a strategy pioneered, naturally, by the Republicans.

“I know from experience that, over a 30-year arc, rightwing conservatives came to dominate American political discourse in the media, and it needs to be countered,” Brock told the Observer last week. “And I know how something like it would work on the progressive side.”

In the culture wars of the mid-90s, Brock, 52, was a far-right hero. He wrote a book casting doubt on the credibility of Anita Hill, the aide who accused supreme court justice Clarence Thomas of sexual harassment. Then, as part of the conservative-funded Arkansas Project, Brock broke the story of “Troopergate” and identified a woman named Paula, aka Paula Jones, one of a string of Bill Clinton “bimbo eruptions” that would culminate in the Monica Lewinsky-inspired impeachment hearings.

Once so committed that his answering machine message said: “Hello, I’m out trying to bring down the president,” Brock has turned on his deep-pocketed former sponsors with a vengeance. First came his sympathetic biography of Hillary Clinton in 1996, followed a year later by an Esquire magazine essay, Confessions of a Right-Wing Hitman, that announced his break with the right. Brock recalled last week how in the mid-90s he began to have “huge reservations about the character and integrity of the people in the conservative movement”.

Everyone loves a sinner redeemed, and Brock is no exception. What he offers is not an ideological or political solution but a willingness to counteract a Republican political machine calibrated to find and exploiting Democrat weaknesses.

In 2004, Brock founded Media Matters for America, a liberal watchdog that helped to bring down Glenn Beck, a Fox News host given to hysterical outbursts, and later helped to publicise comments about “legitimate rape” made to a Missouri TV station by Republican Senate candidate Todd Akin.

He then established American Bridge, a political action committee that has raised $12m from donors including George Soros over the past two years. With more than 80 staff, a key part of its mission is to assign people called “trackers” to tail Republicans, looking for “gotcha” moments that could derail their political ambitions.

Other weapons in the Brock arsenal include the theoretically non-partisan corruption watchdog, Citizens for Responsibility and Ethics in Washington (Crew); the American Democracy Legal Fund, charged with battling Republicans in the courts; and the American Independent Institute, which provides funds for journalists investigating rightwing activities.

Liberals, he says, have failed to understand that political campaigns are constant, not only fought during election years, and require long-term funding. “We’ve basically been trying to play catch-up. There was a tendency on the progressive side to dismiss rightwing media like Fox News as not credible and therefore not important, and they were very late in understanding the nature and power of the infrastructure the rightwing had built.”

While there is residual unease among some liberal operatives that Brock’s conversion story fits into a pattern of opportunism and self-promotion rather than ideological transformation, Brock’s war on the billionaire industrialists Charles and David Koch, coupled with rigorous defence of Hillary Clinton, has earned him growing influence in progressive circles.

Brock acknowledges only that his mission is to counter rightwing attacks, though the focus of those attacks – and thus the rapid-response resources of American Bridge – are clearly centred on preventing opponents from defining Clinton during her candidacy-in-waiting. The left-leaning publication the Nation recently described Brock’s political apparatus as designed “to put Hillary in the White House”.

That unnerves some party advisers who fear this kind of surveillance can only harm the political process. Candidates will be forced to the centre of political discourse. Surrendering principles for electoral success could turn out to be a hollow victory – or no victory at all, says a former Kennedy adviser, Andrew Karsch. “Democrats need a statesman who can articulate the issues, not someone who holds their finger to the wind on every issue. Instead of arguing something, you just mud-wrestle? That’s not an answer. It’s a complete capitulation.”

Despite Brock’s expertise, Democrats may be unsuited to adopting the well-honed tactics of Republicans. This month Democrat billionaire Tom Steyer poured tens of millions into candidates promoting climate-change awareness – a counter to Tea Party funders the Koch brothers – and received no electoral return on his outlay.

Earlier this year, Brock was invited to Arkansas to deliver an address, Countering the Culture of Clinton Hating, to the Clinton School of Public Service in Little Rock. He spoke of how conservatives “upended many of our long-held ways of conducting politics” and how, unless those dynamics are challenged, history could repeat itself.

Of course, it was pursuing that agenda that gave Brock his start. Several years later, Hillary Clinton distributed copies of Brock’s 2002 book, Blinded by the Right: The Conscience of an Ex-Conservative, as proof of the “vast rightwing conspiracy” that had existed against the couple.

In addition to his other groups, Brock serves as an adviser to the grassroots outreach programme Ready for Hillary, and is on the board of Priorities USA, a fundraising operation devoted to a Clinton candidacy. He says Media Matters is already responding to “a fair amount of Clinton-related material”, while American Bridge has a group, Correct the Record, that is solely focused on defending her record. “We’re doing that because there are 10 Republican super PACs [political action committees] out there trying to tarnish her reputation in advance of her making a decision on whether she is going to run,” Brock says. “We already have our hands full in terms of media misinformation.”

He anticipates a silver lining to the Democrats’ recent poor showing at the ballot box. When the newly-elected Republicans start showing their true political colours, his group will be there to document them, and perhaps to influence Republicans’ choice of presidential candidate in 2016. Brock says American Bridge has identified 20 potential Republican candidates for president or vice-president, and put field trackers on them. “We’re way ahead of the curve,” he enthuses. “Based on our research, we’re going to be an important player in how the Republican presidential ticket is defined.”