Elections
On October 2, 2019, Auchincloss announced his candidacy for the open Massachusetts's 4th congressional district to succeed Joe Kennedy III, who unsuccessfully ran for the Senate against incumbent Democrat Ed Markey.
Auchincloss raised the most money during the primary election in both the fourth quarter of 2019 and the first quarter of 2020 and earned endorsements from the National Association of Government Employees, VoteVets, The Boston Globe and James E. Timilty. He earned the support of several Newton politicians, including the president and vice president of the city council and the chair and vice chair of the school committee. He earned additional endorsements throughout the district.
During the campaign, questions arose about his party affiliation. Auchincloss was originally a Democrat but was a registered Republican from 2013 to 2014 while working for Charlie Baker's gubernatorial campaign. He continued to vote in Republican primaries as an independent until late 2015 before becoming a Democrat again.
Auchincloss faced some controversies throughout the campaign and apologized for his old statements that defended the harassment of Black students with a Confederate flag for protecting Newton's free speech values and compared it to banning a pride flag or Black Lives Matter banner, appeared to justify the burning of the Quran, for making fun of a local community efforts' to rename Columbus Day "Indigenous Peoples' Day" in 2016 and he voted against a symbolic 1 percent decrease in the local police budget. He modeled himself after the moderate Republican Governor Charlie Baker.
The Democratic primary took place on September 1, 2020. In a race with eight other candidates, Auchincloss won with 22.4% of the vote. It took the Associated Press three days to call the race because nearly one million votes were cast through mail-in ballots due to the COVID-19 pandemic.
In the November general election, Auchincloss defeated Republican nominee Julie Hall by 21.9% margin while the district voted for Joe Biden by 30.8% margin in the 2020 United States presidential election on the same ballot. He assumed office on January 3, 2021.
Auchincloss declared his intent to run for reelection in September, 2025, after speculation that he would challenge incumbent Ed Markey for the Democratic nomination in the 2026 United States Senate election in Massachusetts. He defeated Jason Poulos, an AI researcher, in the Democratic primary.
Opposition to his campaign accepting $25,000 in donations from Palantir Technologies, including $7,000 from Alex Karp, Palantir's CEO, led to demonstrations outside his Newton office.
