Rep. Justin Amash doubled down Monday on his contention that President Trump has taken actions that meet the threshold for impeachment, rebutting his critics hours after a primary challenger announced a bid to unseat the libertarian-leaning Michigan Republican in 2020.

In the wake of special counsel Robert S. Mueller III’s report on Russian interference in the 2016 campaign, some of Trump’s defenders have contended that the president could not have obstructed justice because there was no underlying crime. In a lengthy Twitter thread, Amash took issue with that defense, arguing that “many crimes” were, in fact, revealed by the investigation and that obstruction does not require the prosecution of an underlying crime to begin with.

“If an underlying crime were required, then prosecutors could charge obstruction of justice only if it were unsuccessful in completely obstructing the investigation. This would make no sense,” Amash said.

Monday’s tweets came after Amash parted ways with the rest of his party Saturday when he accused Trump of having taken impeachable actions. Trump responded by calling the five-term congressman a “loser” and “total lightweight.”

Amash has not ruled out challenging Trump in 2020 on the Libertarian ticket. Now, he has a challenge of his own: On Sunday, an intraparty rival, Michigan state Rep. Jim Lower, declared he would run in the Republican primary next year for Amash’s seat.

Amash has previously said he enlists social media to do something unusual: explain why he bucks his party in terms his constituents can understand.

The 39-year-old Republican, who was the youngest member of the freshman class in 2011, has stated his approach succinctly: “I defend liberty and explain every vote here.” The simple formula has turned Amash, who rode into the House on the tea party wave, into something of a sensation. In 2012, the Independent Voter Network called Amash the “coolest member of Congress.”

But Amash’s latest decision could be difficult for him to explain to Republican voters, 9 in 10 of whom oppose impeachment, according to a Washington Post-ABC News poll conducted in April.

Meanwhile, the move brought praise from Democrats. “You are putting country first, and that is to be commended,” wrote Rep. Rashida Tlaib of Michigan, inviting Amash to co-sponsor her impeachment resolution.

The state’s congressional districts are set to be redrawn before 2020, making his reelection prospects difficult to gauge. The district currently covers Grand Rapids and its exurbs.

Lower announced his challenge to Amash in a release on Sunday.

“I am a Pro-Trump, Pro-Life, Pro-Jobs, Pro-2nd Amendment, Pro-Family Values Republican,” he said. “Justin Amash’s tweets yesterday calling for President Trump’s impeachment show how out of touch he is with the truth and how out of touch he is with people he represents.”

The state lawmaker, who became a county commissioner in 2011 at the age of 21, accused Amash of joining hands with “radical liberals” — naming Tlaib in particular — “to try and bring down our President.” On most issues, a chasm separates Amash from the liberal firebrand.

Lower said that his candidacy had been in the works for two months and that he had been planning to announce around July 4. He sped up his timetable in response to Amash’s tweets. In his announcement, he touted his working-class roots, saying he had put himself through college at Michigan State University and business school at Grand Valley State University by working full-time.

In an interview with The Washington Post, the state lawmaker said he had not read Mueller’s report “in as much detail as Congressman Amash” but agreed with Trump’s claim that there was “no collusion, no obstruction.”

On Sunday, Lower changed his profile picture on Facebook to an image of himself standing in front of a Trump 2020 banner.

Amash has stood under a different banner. At the top of his Facebook page, he has pinned a warning from President George Washington about the “baneful effects” of party loyalty, delivered in his farewell address in 1796.

The Michigan congressman was born in Grand Rapids and raised in Kentwood, Mich., by a Palestinian Christian father and a Syrian Christian mother. He attended college and law school at the University of Michigan and briefly worked at his family’s business, Tekton Tools, which makes handheld implements, such as screwdrivers.

He served one term in the state House in Lansing before he was elected to Congress in 2010. There, he allied with hard-line conservatives to form the House Freedom Caucus, whose members have mostly fallen in line with Trump.

A darling of conservative and libertarian groups, Amash has only once faced a credible primary challenge. He beat his Republican rival, Michigan businessman Brian Ellis, in 2014 by nearly 15 percentage points. Trump won the district handily in 2016, even flipping one county that had been secured by Barack Obama in both 2008 and 2012.

The congressman’s survival may depend on whether he has cultivated devotion among voters sufficient to override their loyalty to the president. His political prospects run through Facebook, where he has labored to convince constituents that he is accountable to them at a time when public trust in government is mired near historic lows.

His heterodox moves have been the ones requiring most careful explanation on social media. He bucks his party in not voting for measures he supports, such as approval of the Keystone XL pipeline or the preservation of Immigration and Customs Enforcement, because he disagrees with the underlying legislative approach.

Social media, the very tool used by Trump in 2016 to bypass media and party gatekeepers and speak directly to voters, could be a bulwark against the president’s attacks. It could help test whether Republicans can break with their standard-bearer and survive. Each Facebook post, in which voters add comments and raise questions, serves as a miniature, and digitized, town hall. Amash has more than 136,000 followers on the platform, about one-fifth of the total number of people 16 and over in his district, according to census estimates.

“Social media is revolutionizing government and making Congress more accountable,” Amash said in 2012, when he emerged as a winner of a Republican new media challenge. “I use Facebook to explain every vote I take on the House floor, and I personally interact with my constituents online. This not only empowers people at home but also shapes me into a better Representative.”

The social networking site has also been a powerful tool of personal branding, helping Amash earn his nickname, “Mr. No,” based on the sobriquet “Dr. No” embraced by Ron Paul, the former Texas congressman whose small-government philosophy the Michigan lawmaker shares.

Amash uses Facebook to make “Star Wars” jokes and flaunt his literacy in emoji. Some of his posts are personal, thanking God for his family or extolling the memory of Friedrich von Hayek, the Nobel Prize-winning economist and apostle of the free market.

But mostly, he explains his “no” votes. No to executive action on immigration. No to federal aid to Flint, Mich., during the height of the water contamination crisis. No to budget plans to keep the government open. No to the National Security Agency’s dragnet collection of phone records.

He even declines to approve the record of House proceedings, protesting, “we are never given adequate time to review the Journal.”

On the question of impeachment, however, Amash answered in the affirmative. Not only did the president’s conduct meet the constitutional standard for impeachment, he argued, but the attorney general, William P. Barr, had “misled the public” about the special counsel’s findings as they pertained to Trump’s possible “high Crimes and Misdemeanors.”

“While impeachment should be undertaken only in extraordinary circumstances, the risk we face in an environment of extreme partisanship is not that Congress will employ it as a remedy too often but rather that Congress will employ it so rarely that it cannot deter misconduct,” the congressman wrote.

Instantly, voters took to the congressman’s Facebook to register their reactions, both positive and negative. He was by turns lauded as an independent thinker, with the best interests of the country at heart, and castigated as a turncoat, interested only in attention.

A professor at Oakland University in Rochester, Mich., thanked him for representing the state with “honesty and honor.” Another woman wrote, “Thank you so much for the courage to speak the truth.”

But he was also told, “You’re fired,” and instructed by an incensed Republican to, “Register immediately as a Democrat and turn in your man card.” Several critics invoked his father’s Palestinian heritage in questioning his loyalty to the Trump and to the United States.