WASHINGTON (Reuters) - U.S. Representative Justin Amash, the first Republican in Congress to say President Donald Trump committed an impeachable offense, quit the Republican Party on Thursday with a blistering attack on politics that puts party over principle.

The congressman from Michigan, who drew a barrage of criticism from fellow Republicans after he laid out a case for Trump’s impeachment, said he has become disenchanted with a political system that is “trapped in a partisan death spiral.”

“The two-party system has evolved into an existential threat to American principles and institutions,” he wrote in a Washington Post opinion piece published on the U.S. Independence Day holiday.

“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,” Amash wrote.

A member of Congress since 2011, Amash, 39, made some high-profile decisions in recent months that chronicled his growing disdain for Trump and subsequent drift from the party.

It was his decision to speak out on Special Counsel Robert Mueller’s report on Russian activities during the 2016 presidential election that drew the president’s fury - and a prompt Republican primary challenge.

Amash said in May that the Mueller report showed Trump had obstructed justice, bucking his party and joining Democrats in castigating the president for his actions. “President Trump has engaged in impeachable conduct,” he said.

Amash’s comments on the Mueller report echoed the conclusions of many Democrats, but Democrats are divided about impeachment. Most Republicans are still standing by the president at a time of economic growth, turbulent markets and global trade tensions.

Trump quickly returned fire by calling Amash a lightweight and a loser. Soon after, Jim Lower, a Michigan state legislator who described himself as “pro-Trump,” said he would challenge Amash in the 2020 congressional race in Michigan, a state Trump won in 2016.

The president welcomed Amash’s news with an acrimonious personal attack on Twitter that also referenced Amash’s primary challenge. “Great news for the Republican Party as one of the dumbest & most disloyal men in Congress is ‘quitting’ the Party,” he said.

Amash has signaled he would consider running as a libertarian against Trump in 2020, but he made no mention of that Thursday. Amash’s office did not return calls for comment.

The leading Democrat to challenge Trump in 2020, former U.S. Vice President Joe Biden, was asked about Amash’s departure while campaigning in Iowa. “Where there’s faith, there’s hope,” he quipped.

In his opinion piece, Amash said he believed that most Americans are not rigidly partisan and do not feel well represented by either party.

In February, he became the lone Republican to co-sponsor a resolution in the U.S. House of Representatives to reject the emergency Trump declared at the U.S.-Mexico border to build a wall there, in a stinging reprimand to the president.

Last month, Amash left the conservative House Freedom Caucus he helped found, whose members usually defend Trump. A libertarian who has targeted government overspending, his July Fourth announcement cast a wide invitation.

“No matter your circumstance, I’m asking you to join me in rejecting the partisan loyalties and rhetoric that divide and dehumanize us,” Amash said. “If we continue to take America for granted, we will lose it.”