When Vermont's Democratic socialist Sen. Bernie Sanders launched his presidential bid with an anti-corporate message, he was tapping into growing populist sentiment among Democrats not quite satisfied with Hillary Clinton. But there is also an emerging populist current among Republicans — only it's manifesting itself in several different ways.

At the heart of the issue is that Republicans have tended to lose campaigns when the perception is that they're out of touch with the concerns of working-class Americans and merely exist to help the rich. Examples of this are George H.W. Bush losing his re-election bid during a mild recession (famously looking at his watch during a town-hall-style debate and bungling a question about how the deficit affected him personally) and Mitt Romney coming across as an aloof multimillionaire (who wrote off 47 percent of the country).

Given that Clinton will be challenged to maintain the same coalition of voters that propelled President Obama to the presidency, she'll have to make up ground among middle class voters — and any Republican who hopes to win in 2016 will have to prevent that from happening.

As a result, Republican candidates are embracing three different philosophical strains of populism.

One brand of populism blends social conservatism with a more expansive view of government.

Mike Huckabee, the evangelical pastor-turned-politician, raised taxes and increased spending as governor of Arkansas. He routinely slams not only Wall Street but also economic conservatives. In his current bid for the White House, he's attacked Republican efforts to reform entitlements and has portrayed himself as a guardian of Social Security and Medicare.

Rick Santorum is also staking out a similar space to Huckabee. Though, to be fair, he has a better record on taxes and fought for Social Security personal accounts. As senator he ultimately embraced George W. Bush's "compassionate conservative" agenda — voting for the Medicare prescription drug program and No Child Left Behind. Though he's since called those votes a mistake, even recently, he's come out in favor of increasing the minimum wage.

Also fitting into this group, should he run, is Ohio Gov. John Kasich, who has framed his support for expanding Medicaid through Obamacare as an expression of his Christianity.

A second strain that has gained steam has been called libertarian populism. For years, thinkers on the right such as my colleague Tim Carney have been raising alarms about the ways in which big business works to expand government. The idea is that the goal of public policy shouldn't be to be "pro-business" as much as it should be pro free market. This translates into attacking taxpayer giveaways to those corporations that can hire the best lobbyists.

Traditionally Republicans have suffered from being cast as being on the side of big business due to their stances on reducing taxes and regulations. But libertarian populism offers Republicans an opportunity to be on the side of the little guy while still advocating a pure free market approach.

This theme has now become more popular among Republican presidential candidates, who have blasted crony capitalism including the Export-Import Bank (which theoretically exists to promote U.S. exports, but effectively subsidizes major corporations such as Boeing).

Sen. Ted Cruz, R-Tex., has described himself as a foe of crony capitalism and even came out against ethanol subsidies in Iowa. Former Hewlett-Packard CEO Carly Fiorina, who also criticized ethanol subsidies, has pointed out how lobbyists write regulations to benefit big companies at the expense of small business. "The more complicated government gets — and it's really complicated now — the less the small and the powerless can deal with it," she has said. Louisiana Gov. Bobby Jindal has highlighted the role that hospitals have played in getting states to participate in Obamacare's Medicaid expansion.

On top of this, there's a third strain of populist protectionism that has manifested itself on trade and immigration. Sen. Jeff Sessions, R-Ala., has criticized immigration policies and trade deals for not thinking enough about their impact on the American worker, and he's having influence.

After meeting with Sessions, Walker, who previously supported a pathway to citizenship for illegal immigrants, has started talking about limits on legal immigration. "In terms of legal immigration, how we need to approach that going forward is saying — the next president and the next congress need to make decisions about a legal immigration system that's based on, first and foremost, protecting American workers and American wages," he said, causing a political firestorm.

On trade, Huckabee has lamented, "Washington looks the other way and ignores the law so we can import low-wage labor, undercut American workers, and drive wages lower than the Dead Sea."

The lines on these different variations of populism aren't firmly drawn. For instance, Huckabee is not beyond railing against crony capitalism. Walker has also talked about his faith, but isn't going to use it to argue for expanding Medicaid.

But it's pretty clear that Republicans have realized that they have to find some way of connecting with working-class voters, and they are testing out multiple avenues of getting there.