During his time in office, Tester voted for the Economic Growth, Regulatory Relief, and Consumer Protection Act, which rolled back parts of the Dodd–Frank Act, and joined Republicans in supporting a measure to delay certain environmental regulations affecting coal power plants.

He voted against the DREAM Act and against Democratic proposals to expand background checks, and has supported efforts to loosen restrictions on gun exports.

[6] Tester grew up in Chouteau County, near the town of Big Sandy, Montana, on land that his grandfather homesteaded in 1912.

[9] Tester then worked for two years as a music teacher in the Big Sandy School District before returning to his family's farm and custom butcher shop.

[11] Tester spent five years as chairman of the Big Sandy School Board of Trustees and was also on the Big Sandy Soil Conservation Service (SCS) Committee and the Chouteau County Agricultural Stabilization and Conservation Service (ASCS) Committee.

[15] Tester cited a prescription drug benefit program, reinstatement of the "Made in Montana" promotion program, a law to encourage renewable energy development, and his involvement with a bill that led to an historic increase in public school funding as accomplishments while in office.

Tester had more support from his fellow legislators,[23] but Morrison, whose grandfather was governor of Nebraska, raised significantly more money and had greater statewide name recognition.

[27] While Tester's pledge to "end secret meetings with lobbyists" was a central issue in his campaign, CNN reported in 2023 that he had not fully followed through on it.

[32] When announcing his candidacy, Rehberg called Tester a "yes man" for President Obama, saying that he sided with the administration in 97% of his votes.

The Los Angeles Times noted that Tester diverged from his party on matters such as gun rights and illegal immigration.

[36] President Donald Trump made a particular effort to unseat Tester, traveling to Montana four times over the preceding months.

Tester made some moves to distance himself from the Joe Biden administration, but his voting record remained in line with the Democratic Party.

[50] He called the storming a "despicable and dangerous attack on our democracy" and "a coup by domestic terrorists",[51][52] and blamed Trump for instigating it.

[54] A New York Times profile of Tester after his 2006 election described him as "truly your grandfather's Democrat—a pro-gun, anti-big-business prairie pragmatist whose life is defined by the treeless patch of hard Montana dirt that has been in the family since 1916".

[55] In 2012, USA Today noted that Tester had sometimes "split with Democrats—most recently in his support of construction of the Keystone XL oil pipeline from Canada to the Gulf Coast—but he has voted with Obama on the most critical issues of his presidency: the stimulus, the health care legislation and the Dodd-Frank financial services overhaul".

The New York Times wrote that his "electoral successes trace back to carefully tailored campaigns that catered to local issues over dominant national ones like abortion", and that for red state Democrats like Tester and Sherrod Brown of Ohio, it was an open question whether they could "maintain their invaluable political personas while—for the first time in their lengthy careers in public office—persuading their constituents to keep abortion rights front and center when voting next year [in 2024].

[62] Tester became one of the Democrats in the Senate to support the 2018 Economic Growth, Regulatory Relief and Consumer Protection Act, a bill that partially repealed Dodd-Frank and relaxed key banking regulations.

As one of at least 11 other Democrats, he argued that the bill would "right-size post-crisis rules imposed on small and regional lenders and help make it easier for them to provide credit".

[64] A Newsweek reporter who traveled with Tester in Montana in 2011 said that the "desire to wrest control of wolves from D.C. ... was the only topic that came up everywhere he went: hotels, coffee shops, art auctions.

The bill would put 700,000 acres of wilderness aside for "light-on-the-land logging projects" with the intention of creating jobs in the flagging industry.

It was noted that Tester was not "winning admirers on his side", with some liberal environmentalists saying that would give lumber mills control of the national forests.

He argued that the bill would "have blocked family members and neighbors from buying and selling guns to one another without a background check".

Tester voted for a second Democrat-sponsored proposal to ban gun sales to people on the terrorist watch list.

[75] In 2010, Tester voted against the DREAM Act, which would have created a pathway to citizenship for the foreign-born children of illegal immigrants.

[82] While he opposed same-sex marriage during both his 2006 and 2012 campaigns, Tester announced his support for it in 2013, citing concerns about federal government overreach.

"[86] He opposed the confirmations of Jeff Sessions as Attorney General,[87] Mike Pompeo as Director of the Central Intelligence Agency,[88] and Neil Gorsuch as Associate Justice of the Supreme Court of the United States for supporting the PATRIOT Act's bulk data collection provisions.

Among other reasons, he cited "concerns that Judge Kavanaugh defended the PATRIOT Act instead of Montanans' privacy", as Kavanaugh had helped the Bush administration craft a program of mass domestic surveillance and had ruled in favor of increased government surveillance under the PATRIOT Act in Klayman v.

[91] Tester opposed the Supreme Court decision Citizens United, which allows corporations and unions to donate unlimited amounts of money to third-party political groups.

[99] The first Democrat from a red state to express opposition to her, he cited her role in Bush administration interrogation and detention programs, and said he was "not a fan of waterboarding".

[100] According to CNN, four sources familiar with the allegation that Jackson drunkenly banged on the door of a female colleague confirmed it.