WASHINGTON (AP) — He was known in the Michigan statehouse as "Mr. No" for voting against some Republican legislation. But now in Congress, on the question of whether President Donald Trump should be impeached, Rep. Justin Amash is the lone Republican saying "Yes."

In tweeted remarks over the weekend, Amash wrote that he's read special counsel Robert Mueller's report on Trump's conduct during and after the 2016 presidential election. Mueller did not find evidence of conspiracy with Russia, but he revealed startling details about Trump's efforts to shut down the probe and made no recommendation on obstruction. Amash did, becoming the only Republican in Congress to call for the House to formally charge the president.

"Mueller's report reveals that President Trump engaged in specific actions and a pattern of behavior that meet the threshold for impeachment," Amash, a longtime Trump critic, tweeted on Saturday after reading the report. Specificially, Amash tweeted, the findings identify "multiple examples of conduct satisfying all the elements of obstruction of justice."

The backlash from Republicans was swift and sharp against a congressman from a key state that Trump swiped from Democrats — by less than a percentage point — for the first time since the Reagan administration. In his bid to keep the state in his column next year, Trump launched a weaponized tweet that could serve as a warning to other Republicans considering defying him. None immediately did so.

Within hours, Trump had called Amash "a total lightweight" and a "loser." House Republican Leader Kevin McCarthy on Fox News questioned whether Amash really belongs among GOP congressmen. But most notably, the fifth-term lawmaker drew a new primary challenger for his Grand Rapids-area seat in the 2020 elections. The key disqualification, as they described it across the board Monday, was Amash's insufficient loyalty to the president on matters ranging from Trump's wall to Mueller's "witch hunt."

"Amash has consistently voted against President Donald Trump on important issues," said state Rep. Jim Lower, who scrambled to announce his campaign Monday after Amash's tweets. In a telephone interview later in the day, Lower said, "Most Republican Party primary voters support the president and want a congressman that would work with him to get his agenda done." He accused Amash of "standing in the way" and said the congressman has moved away from the party over several years.

Amash was elected in 2010 as part of the tea party wave that toppled Democratic control. He was one of the founding members of the conservative House Freedom Caucus. He vowed to explain all of his votes, and to never miss one.

But many of the Freedom Caucus members are no longer in Congress and the group is now dominated by pro-Trump Republicans. Amash acknowledges that he's somewhat isolated in Washington as a result, describing himself in the "Liberal Lions" podcast in March as "not personally lonely, but politically lonely."

Back home in Michigan, establishment Republicans in the business community have long been disenchanted with Amash, saying he does not do enough to solve problems in the district. They backed an unsuccessful primary challenger in 2014. But the dynamics have shifted with Trump's election, and GOP operatives say the president's criticism of Amash has hurt the congressman's standing with the base.

"He's the most vulnerable in a Republican primary that he's ever been," said Greg McNeilly, a Republican strategist in Grand Rapids. "The 3rd District, while much more of a swing district in a general election for the president, is a solidly Trumpian district in a primary. The delta between the president and congressman is the chief source of Justin Amash's problem."

Like all Republicans, Amash is well aware of the potential cost of defying Trump. Unprompted in the same podcast interview, he harked back to Mark Sanford, the former North Carolina governor and congressman critical of Trump who became the focus of the president's twitter fury — and lost the GOP primary ahead of the 2018 elections. A Democrat ultimately won that seat.

So it may come as no surprise that Amash isn't ruling out challenging Trump on the Libertarian ticket in 2020, in part because he wants to offer people an alternative to the major parties.

For certain, he wasn't backing down on Monday. At midday, Amash answered his critics on legal and constitutional grounds, tweeting, among other things, that there need not be a statutory crime named in the Mueller report for impeachment to be appropriate.

In the nearly two dozen-tweet stream throughout Saturday and Monday, Amash kept one scolding quote pinned at the top.

"'Let me now...warn you in the most solemn manner against the baneful effects of the spirit of party generally.' - George Washington."

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Associated Press Writer David Eggert in Lansing, Michigan contributed to this report.

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