WASHINGTON - Justin Amash, the only U.S. Republican lawmaker who has called for President Donald Trump's impeachment, quit the party on Thursday, Independence Day in America, to become an independent.

Amash, a 39-year-old, five-term congressman from the Midwestern state of Michigan, said in a Washington Post opinion article that the country's two-party political system is broken, with modern politics "trapped in a partisan death spiral" between warring Democrats and Republicans.

Amash angered Trump and Republican colleagues in Congress when he recently called for Trump's impeachment, based on allegations that Trump obstructed justice by trying to thwart special counsel Robert Mueller's investigation into Russian meddling in the 2016 presidential election. Amash now becomes the only independent lawmaker in the 435-member House of Representatives.

After Amash's defection from the Republicans, Trump launched a new attack on him.

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'Great news for the Republican Party as one of the dumbest & most disloyal men in Congress is 'quitting' the Party," Trump said on Twitter. "No Collusion, No Obstruction! Knew he couldn’t get the nomination to run again in the Great State of Michigan. Already being challenged for his seat. A total loser!"

In the newspaper article, Amash decried what he said are the "consequences of a mind-set among the political class that loyalty to party is more important than serving the American people or protecting our governing institutions. The parties value winning for its own sake, and at whatever cost. Instead of acting as an independent branch of government and serving as a check on the executive branch, congressional leaders of both parties expect the House and Senate to act in obedience or opposition to the president and their colleagues on a partisan basis."

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He concluded, "Most Americans are not rigidly partisan and do not feel well represented by either of the two major parties. In fact, the parties have become more partisan in part because they are catering to fewer people, as Americans are rejecting party affiliation in record numbers."

"Preserving liberty means telling the Republican Party and the Democratic Party that we’ll no longer let them play their partisan game at our expense," he said. "Today, I am declaring my independence and leaving the Republican Party. No matter your circumstance, I’m asking you to join me in rejecting the partisan loyalties and rhetoric that divide and dehumanize us. I’m asking you to believe that we can do better than this two-party system — and to work toward it. If we continue to take America for granted, we will lose it."

At least 80 of the 235 Democrats in the House have called for Trump's impeachment or an inquiry that could lead to his impeachment. But House Speaker Nancy Pelosi, the leader of the Democratic-controlled chamber, has resisted the call for an impeachment inquiry while supporting multiple committee investigations into Trump and his presidency.

Mueller concluded that Trump did not conspire with Russia to help him win the election three years ago, but reached no conclusion whether, as president, he obstructed justice. Subsequently, however, Attorney General William Barr and then Deputy Attorney General Rod Rosenstein decided that obstruction charges were not warranted against Trump.

Mueller is set to testify about his 22-month investigation before two House committees on July 17.