Julian Zelizer is a professor of history and public affairs at Princeton University, and author, with Kevin Kruse, of the new book "Fault Lines: A History of the United States Since 1974." Follow him on Twitter at @julianzelizer. The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) Following Robert Mueller's statement in May spelling out the conclusions of his report and rejecting the president's accusations about the origins of the special counsel's investigation, Republican Rep. Justin Amash tweeted the most pertinent point: "The ball is in our court, Congress."

The tweet came the morning after Amash returned to a hero's welcome at a town hall Tuesday in Grand Rapids, Michigan. After sending out a series of Twitter threads explaining why President Donald Trump committed impeachable offenses and why Attorney General William Barr covered up the findings of Mueller's report, Amash has emerged as the singular voice of genuine GOP opposition to the President.

Much more representative of the party as a whole is the approach taken by Senate Majority Leader Mitch McConnell, Barr and Sen. Lindsey Graham, who on this week's The New Yorker cover are depicted shining Trump's shoes: Establishment Trumpism is all the rage.

But Amash doesn't seem to care. When the President has attacked Amash, the congressman has doubled down and gone after him even harder. His arguments offer the kind of clearheaded and pointed statements that Democrats had been hoping to hear from their own leadership.

Amash's constituents are used to his being a rebel . This libertarian conservative, elected as part of the Tea Party movement in 2010, is no stranger to mixing with his own. Though he is extremely conservative, including on issues such as abortion, he has been willing to take on the GOP on matters such as surveillance.

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