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Justin Amash could derail the White House ambitions of either party. Here's how to handle that.

Editor's Note: (Kurt Couchman served in the Washington, DC, office of Representative Justin Amash from 2011 to 2015. He now conducts government affairs for a nonprofit advocacy organization. The opinions expressed in this commentary are his. Read more opinion articles on CNN.)

(CNN) President Donald Trump and other prominent politicians summarily dismissed Rep. Justin Amash's declaration of independence from the Republican Party. That's a mistake. Amash could upend the 2020 presidential contest and congressional elections as a third-party presidential candidate, and neither party is prepared for that eventuality.

Those who don't know Amash usually misunderstand and underestimate him. But having served as a policy adviser in his congressional office for four years, I don't. If he gains third-party backing for a presidential race, he would likely have a massive impact.

Kurt Couchman

How could a much lesser-known candidate possibly compete with the Republican and Democratic electoral juggernauts? His chances of winning the presidency are vanishingly small. But he could attract votes that would have otherwise gone to the Republican and Democratic nominees, thus complicating the race enormously. That is, unless our system is reformed to let voters rank their preferences -- and avoid a spoiler situation.

As a constitutionalist, consistent proponent of liberty, devoted family man, and faithful Christian, the now-former Republican could easily attract votes from otherwise reluctant Trump voters. And in competitive states like Pennsylvania, Wisconsin and, of course, Michigan, small margins make a big difference. Also, he would draw votes from Democrats. He opposes corporate welfare and other forms of corruption, he vocally backs civil liberties against both parties' leadership, he stands against global militarism, and he frequently engages bipartisan coalitions to advance reforms.

If Democrats nominate a far-left, socialist sympathizer, Amash's commitment to fiscal responsibility, coalition-building, and the public interest over special interests could easily draw support from disaffected Democratic voters.

Of course, Amash certainly wouldn't have equal resources. Even so, backing from the Libertarian Party would provide a strong tailwind. In 2016, the party secured ballot access in all 50 States and DC for nominee Gov. Gary Johnson. Johnson never cleared the 15% threshold in five nationwide polls to participate in the presidential debates, but he still set a record for the party with 3.27% of the popular vote.

Amash has his own significant strengths. He is among the most thoughtful and careful members of Congress, and his ability to resist political pressure is legendary. From the beginning of his time in Congress, he has applied a faithful understanding of the Constitution and rigorous policy analysis -- supported by my colleagues and me -- to every question that came before him. Now in his ninth year in the House, he has developed informed views on a wide range of federal issues. He would be formidable and articulate in debates, and he has the social media acumen to reach the people directly.

If he, like Johnson, tapped someone like former GOP Gov. Bill Weld, currently a Republican challenging Trump for the GOP nomination, as running-mate, the experience of a statewide chief executive would add depth and balance.

Republicans and Democrats do, however, have an opportunity to keep Amash from blowing up the race and weakening the ultimate winner. That option is called "ranked-choice voting," "single-transferrable ballot," or "instant run-off voting." A voter would rank their top two or three choices for the office, and if the first (or second) vote isn't one of the two finalists, the vote transfers to the next choice.

To be specific, a voter may want to back Amash, but she'd vote for Trump to avoid giving the Democrat an advantage. Her friend prefers Trump to the Democrat but dislikes Trump -- or likes Amash -- enough to vote for Amash anyway. Some would break each way. With ranked-choice voting, both could ultimately support Trump without "throwing their votes away." This dynamic applies equally for Democrats.

In either case, if each voter lists a major party candidate, the victor would have the support of at least half of the American electorate. Presidents Bill Clinton, George W. Bush, and Trump's mandates were each weakened by initially winning with only pluralities. We can do better.

Ranked-choice voting would, of course, also reveal more support for minor parties. Far from being a challenge to the viability of the dominant parties, it would help them improve their appeal to more independent voters, thus improving representation for everyone.

Admittedly, only Maine has adopted ranked-choice voting statewide, but a dozen states have experience with it. Nonetheless, the stakes for 2020 are incredibly high, and the American people need clear outcomes.

Enacting ranked-choice voting for selecting a president through the Electoral College is generally a State responsibility (Art. II, Sec. 1), however. For regulating congressional elections, on the other hand, the Constitution empowers States, but Congress is authorized to step in "at any time" (Art. I, Sec. 4). If Congress required this reform for its own elections, most States would likely conform the presidential process in short order, especially as more citizens recognize the benefits.

Neither Republicans nor Democrats know how the 2020 presidential contest will develop. No matter what, if Amash enters the race, the dynamics will change dramatically. To ensure that the ultimate victor has a mandate, however, Congress and the States should enact ranked-choice voting without delay.