Regardless of whether he wins the Republican primary, makes an independent run, or bows out of the 2012 presidential race altogether, this aspect of Ron Paul’s future is indisputable: Come January 2013, he will no longer be a member of the House of Representatives.

Last July, the 12-term Republican representative from Texas, deep in his third bid to gain the Oval Office, announced that he was not standing for re-election.

Paul will leave behind a big pair of shoes to fill when it comes to being an indefatigable champion of reducing the size, scope, and spending of the federal government. No politician over the past several decades has been more outspoken and consistent in his views—and votes—for smaller government.

Even a cursory look at the budget plans being hawked by the three other Republican presidential contenders and by Barack Obama shows just how rare Paul's politics really are.

Who can take Paul's place in the House of Representatives? The 2010 midterms flooded the lower chamber with 94 new members, many of them supported by Tea Party factions. The ideal candidate to take up Paul's libertarian message would oppose the government picking market winners and bailing out market losers; militarization at home and abroad; and economic and social engineering of all stripes.

3.) Rep. John Duncan Jr. (R-Tenn.)

When Paul retires, Tennessee Congressman John "Jimmy" Duncan Jr. will be the last sitting House Republican to have voted against the Iraq War. Not just once, but every time the issue was on the table. Along with Paul, Duncan voted against the war in 2003, against a House declaration in support of the war in 2006, and against further funding for the war in 2007. Also in 2007, Duncan voted in favor of a resolution to withdraw all U.S. troops within 90 days of the resolution’s passage.

What else has Duncan voted against? The Troubled Asset Relief Program (TARP), the American Reinvestment and Recovery Act, making the PATRIOT Act permanent, and ObamaCare. If that sounds a lot like Paul’s voting record, it’s because Duncan, who has served since 1988, is a member of Paul’s Liberty Caucus.

In his own words: “The traditional conservative position on foreign policy is a noninterventionist foreign policy, and Congressman Paul has been a very forceful advocate of that,” Duncan told antiwar.com in 2007. When asked about Paul’s statement during the GOP presidential debates about blowback, Duncan replied, “What [Paul] should have said, in my opinion, is that nothing that we've ever done or not done could ever justify the killing of innocent people such as occurred in New York City. On the other hand, at some point we're going to have to realize that we can't afford to keep getting involved in every religious, ethnic, and political dispute around the world. It's unconstitutional and unaffordable, and it goes against every traditional conservative position I've ever known.”

Why He’s Not Number 1: Despite being against market and military interventionism, Duncan has a mixed record on domestic policy. While he supports vouchers and charter schools, gun rights, and free trade, he also supports a constitutional amendment defining marriage as between a man and woman, drug testing for professional athletes and federal employees, making death penalty appeals harder, and banning various types of political ads. Additionally, Duncan supported Cash for Clunkers and thinks tobacco should be regulated as a drug.

2.) Rep. Jared Polis (D-Colo.)

Ron Paul isn’t the only marijuana reform advocate retiring from Congress this year. Rep. Barney Frank (D-Mass.) is also taking a powder. When it comes to economic regulation and sexual orientation, Jared Polis is more like Frank than Paul, but he shares the latter’s deep concern with civil liberties.

Polis took office in 2009 and his list of libertarian-ish ideas is short but powerful. He’s called for an end to federal marijuana prohibition, says the Iraq War was a colossal mistake, opposes the Stop Online Piracy Act and the Fairness Doctrine, voted against the PATRIOT Act’s warrantless wiretaps, the suspension of habeus corpus for terror suspects, and domestic spying. He also supports travel to Cuba, and even started a nonprofit foundation in Colorado to help launch charter schools. (Contrary to an earlier version of this article, Polis wasn't in office to vote for TARP, but he did oppose the Detroit bailout.) 

In his own words: “Ending the failed policy of prohibition with regard to marijuana will strike a major blow against the criminal cartels that are terrorizing Americans and Mexicans on both sides of the border,” Polis said at a National Press Club Event. “It’s been estimated that the drug cartels derive about half of their revenue from marijuana, so I think it would reduce the violence by half, and reduce the money that fuels the criminal enterprises by half.”

Why He’s Not Number 1: Polis may be good in civil liberties, but he’s a big-government Democrat on absolutely everything else. You name it, Polis has supported it: The pro-union Card Check, a national service initiative, additional  the initial stimulus, environmental restrictions on emissions, $40 billion for “green public schools”—the list goes on.

1.) Rep. Justin Amash (R-Mich.)

Having only taken office in January, Justin Amash doesn’t have much of a voting record to object to. This, combined with his heavy handed social conservatism—he believes life begins at conception and supports the Federal Defense of Marriage Act—might make him a strange choice as the member of Congress most likely to pick up where Ron Paul leaves off.

And yet...

Amash is a vocal proponent of Paul’s small-government philosophy and has endorsed his presidential run. The votes he has cast so far show a profound understanding of the many ways in which the federal government currently infringes on personal liberty. Amash has criticized President Obama's health-care legislation for its deleterious effects on small businesses and defended the right of parents to homeschool their children. He opposes all energy subsidies and most energy regulations. According to his website, Amash supports "lower personal and corporate tax rates," a simpler tax code, and a broader tax base. He's "also open to proposals that tax spending instead of income as a substitute to our current system."

Additionally, Amash supports a balanced-budget amendment, and believes all legislation should have to point to language in the Constitution that proves its legitimacy. While Amash wasn’t in Congress to vote for or against the wars in Iraq and Afghanistan, he did vote to ban armed forces in Libya without congressional approval, and opposed the National Defense Authorization Act (NDAA) as well as many provisions of the PATRIOT Act. The fact that Amash explains every vote he casts on his Facebook page is not simply a pedantic cherry on top: It shows the 31-year-old understands not just 21st-century media but takes seriously the need for a more transparent government.

In his own words: "Our Constitution does not permit the federal government to detain American citizens indefinitely without charge or trial," Amash wrote in a letter that was later signed by six other House Republicans and four House Democrats. "We strongly believe in protecting the country’s security and equipping our Armed Forces with the tools they need to defeat our enemies. But we cannot support measures that, in the name of security, violate Americans’ constitutional rights."

Mike Riggs is an associate editor at Reason. Follow him on Twitter.

Follow Reason on Twitter.

What do you think of our choices? Who would you have picked to succeed Ron Paul as the House of Representatives' leading champion of libertarian policies? Share your thoughts in the the comments.