[4] Formerly the District Attorney for Weld County, Colorado, Buck ran unsuccessfully for U.S. Senate in 2010, losing to Democrat Michael Bennet.

In Congress, Buck joined the Freedom Caucus, and emerged as a staunch fiscal conservative, as well as one of the foremost proponents of antitrust enforcement in the Republican Party.

[5][6][7] Buck announced in November 2023 that he would not seek a sixth House term, stating that his party's "insidious narratives breed widespread cynicism and erode Americans' confidence in the rule of law."

[11] He and his two brothers were encouraged by their parents, Ruth (Larsen) and James Buck, both New York lawyers, to attend Ivy League colleges.

[12][13] Buck earned a Bachelor of Arts degree in politics from Princeton University in 1981 with a 75-page long senior thesis titled "Saudi Arabia: Caught Between a Rock and a Hard Place".

Following that assignment, he worked as a prosecutor with the U.S. Department of Justice in Washington D.C.[16] In 1990, Buck joined the United States Attorney's Office for the District of Colorado, where he became Chief of the Criminal Division.

Buck was formally reprimanded and required to take ethics classes in 2001 for a meeting he had with defense attorneys about a felony case he thought should not be pursued.

[17] Buck said he is "not proud" of the incident that effectively ended his career with the Justice Department,[17] but that he felt it was unethical to prosecute such a "weak" case.

The American Civil Liberties Union sued Buck's office for violating the privacy of the service's clients and after an appeal to the Colorado Supreme Court, costing the county approximately $150,000, the raid was deemed unconstitutional.

In the meeting that she recorded, Buck said, "It appears to me … that you invited him over to have sex with him," and that he thought she might have wanted to file rape charges to retaliate against the man for some bad feeling left over from when they had been lovers more than a year earlier.

Bremer replied: "Uh, yes, sir, I understand the central committee has adopted a resolution that requires me to sign a false affidavit to the state".

[18] Contrasting himself to what he argued was the "top down" style of early Republican favorite Lieutenant Governor Jane Norton, Buck pledged a "bottom-up" campaign that would include visits to each of Colorado's 64 counties.

[18] Initially Norton was seen to have had a nearly insurmountable advantage against "a band of underfunded unknowns" that included Buck, who early in the primary season was called "a dead-in-the-water Republican U.S. Senate candidate with laughable fundraising totals and little establishment GOP support".

He attempted to make a virtue of his meager war chest by positioning "himself as the small-money underdog" in an election cycle that saw a "populist push for outsider candidates to upset the Washington establishment".

Buck blamed the comments on his exhaustion and frustration after months of campaigning, and on his exasperation that it was difficult to keep debate focused on the mounting governmental debt.

[31] (According to a mass email sent on behalf of Senator Jim DeMint, it was a joking paraphrase of his opponent's suggestion to vote for her "because I wear high heels.")

He was also attacked for wanting to eliminate the Seventeenth Amendment[37] and refusing to prosecute an alleged rapist as Weld County district attorney.

[39][40] On August 19, 2013, Buck emailed supporters and announced that the lymphoma he had been diagnosed with was in remission following treatment and he would run against Senator Mark Udall in 2014.

"[52] On March 4, 2020, Buck was one of only two representatives to vote against an $8.3 billion emergency aid package meant to help the United States respond to the COVID-19 pandemic.

"[57] In September 2023, Buck came out against House Speaker Kevin McCarthy's decision to announce an impeachment inquiry into President Joe Biden.

The inquiry was launched by McCarthy to uncover alleged corruption on the part of the President in relation to his son Hunter's business dealings.

[61] In April 2023, Buck was one of only four Republican representatives who voted against the proposed Limit, Save, Grow Act, which raised the debt ceiling while at the same time providing for cuts to non-mandatory spending;[62] he explained in a subsequent Washington Times op-ed that "[i]f this plan with modest spending reductions is the best the GOP can provide, Americans could be forgiven for wondering what the point of a Republican majority in the House is.

He spent most of his three-minute video announcement chastising fellow Republicans for being "obsessively fixated on retribution and vengeance for contrived injustices of the past."

He continued that "Too many Republican leaders are lying to America, claiming that the 2020 election was stolen, describing January 6 as an unguided tour of the Capitol and asserting that the ensuing prosecutions are a weaponization of our justice system."

[71][72] For the 118th Congress:[73] In December 2020, Buck was one of 126 Republican members of the House of Representatives to sign an amicus brief in support of Texas v. Pennsylvania, a lawsuit filed at the United States Supreme Court contesting the results of the 2020 presidential election, in which Joe Biden defeated incumbent Donald Trump.

The Supreme Court declined to hear the case on the basis that Texas lacked standing under Article III of the Constitution to challenge the results of an election held by another state.

[88] Buck favors bipartisan legislation designed to bolster the federal government's ability to bring antitrust cases against "Big Tech" companies.

[4] In 2022, Buck was one of 39 Republicans to vote for the Merger Filing Fee Modernization Act of 2022, an antitrust package that would crack down on corporations for anti-competitive behavior.

[101][102] In September 2021, Buck was among 75 House Republicans to vote against the National Defense Authorization Act of 2022, which contains a provision that would require women to be drafted.

[108][109] Buck's last action as a congressman was to sign discharge petitions that would force votes on bills supplying military aid to Ukraine, Israel, and Taiwan.