Which years did Republicans control both the House and Senate in the 21st century?
Executive summary
Republicans controlled both the House of Representatives and the Senate at several discrete intervals in the 21st century: briefly in 2001, continuously from 2003 through 2006, and again in the two consecutive Congresses spanning 2015–2016 and 2017–2018 (each defined by congressional terms) [1] [2] [3] [4] [5].
1. How to read “control” in this question
Control here means which party held the majority of seats in both the House and the Senate at the same time; this is a narrow institutional definition and does not require the presidency to be of the same party (the sources use the same convention when marking unified or split Congresses) [1] [6].
2. The 2001 brief window: Jan 20–June 6, 2001
After the 2000 elections the Senate was evenly divided, and Republicans did not effectively hold the Senate majority until George W. Bush’s inauguration on January 20, 2001, when Vice President Dick Cheney could break 50–50 ties and Republicans assumed the majority; that Republican Senate majority lasted until Senator James Jeffords’ switch to Independent (caucusing with Democrats) on June 6, 2001, which returned the chamber to Democratic control — during that Jan 20–June 6 period Republicans held both chambers (the House was already majority-Republican) [1] [2].
3. Full congressional terms with unified GOP control: 2003–2006
Following the 2002 midterm elections Republicans entered the 108th and 109th Congresses with clear majorities in both chambers; Republicans controlled both the House and the Senate for the period corresponding to the 108th and 109th Congresses — effectively covering calendar years 2003, 2004, 2005 and 2006 (convening Jan 3, 2003 through Jan 3, 2007) [1] [6].
4. The mid‑2010s Republican majorities: 2015–2016 and 2017–2018
Republicans won both chambers in the 2014 elections, producing unified GOP control during the 114th Congress from Jan 3, 2015 to Jan 3, 2017 — covering calendar years 2015 and 2016 — and they retained majorities in both the House and Senate in the 115th Congress (Jan 3, 2017–Jan 3, 2019), which corresponds to calendar years 2017 and 2018 [3] [4]. The 115th Congress, with the inauguration of President Trump on January 20, 2017, is often noted as a full federal trifecta, but the underlying fact for this question is simply that Republicans held both congressional chambers through those two consecutive Congresses [4] [5].
5. What the sources say about other years and caveats
Authoritative institutional timelines and congressional histories make clear that party control can flip intra‑term (special elections, party switches) and that the technical start/stop of a “period” of dual-chamber control is set by congressional organization dates or official party-caucus shifts — for example, Jeffords’ June 2001 switch is the canonical intra‑term event that ended the early‑2001 Republican Senate majority [2] [1]. Some secondary summaries count whole Congresses rather than calendar-year slices; the answer above reconciles both conventions by citing the exact convening dates and the well-documented intra-term change in 2001 [1] [2].
6. Short synopsis — years listed plainly
Using the institutional dates and accounting for the June 2001 party switch, the calendar years during which Republicans controlled both the House and the Senate in the 21st century are: 2001 (Jan 20–June 6), 2003, 2004, 2005, 2006, 2015, 2016, 2017, and 2018 [1] [2] [3] [4].