May government (2016–2019)
After David Cameron resigned due to the United Kingdom's vote to leave the European Union, the Conservatives had a leadership election in which Rees-Mogg initially supported Boris Johnson. After Johnson chose not to run, Rees-Mogg endorsed Michael Gove and after Gove was eliminated he backed Andrea Leadsom. Leadsom then withdrew, which meant that Theresa May became Conservative leader and Prime Minister.
Rees-Mogg supported the then-Republican Party nominee Donald Trump during the 2016 U.S. presidential election. In October 2016, when the Donald Trump Access Hollywood tape surfaced, he distanced himself from Trump's Twitter messages, saying that Twitter was "fundamentally trivial". In May 2018 he wrote an article for The Times titled 'Trump Will Be Our Greatest Ally After Brexit', saying that he "appealed to voters left behind by the metropolitan elite and he exudes confidence about his own nation and a determination not to be a manager of decline, which also inspires the Brexiteers".
In November 2017, Rees-Mogg met Trump's former White House Chief Strategist and Breitbart News' executive chairman Steve Bannon to discuss how right-wing movements can succeed in the United Kingdom and the United States. Rees-Mogg later defended the meeting when asked about it in an interview, stating, "I've talked to any number of people whose political views I do not share or fully endorse. ... Inevitably politicians meet other politicians. Mr Bannon was the chief of staff to President Trump and is a senior figure in the Republican Party."
In 2017, he supported the confidence and supply agreement made between the Conservative Party and the Democratic Unionist Party (DUP). He later addressed a DUP fundraising event, drawing criticism from the Northern Ireland Conservatives.
Rees-Mogg was widely regarded as a potential candidate for the leadership of his party, something he was reportedly considering during 2017. On 13 August 2017, however, Rees-Mogg said that such speculation was "part of media's silly season". Two Conservative MPs, Heidi Allen and Anna Soubry, announced that they would leave the party if he became leader; another, Justine Greening, suggested she could do the same. However, other Conservative MPs, such as Jesse Norman and Daniel Kawczynski have expressed support for a prospective Rees-Mogg leadership bid. Former UKIP leader Nigel Farage also backed a potential Rees-Mogg candidacy.
Following the 2017 general election, calls were made for Theresa May to step down as Prime Minister and leader of the Conservative Party after failing to win an overall majority in the House of Commons. This led news outlets to begin speculating on May's possible successor with Boris Johnson touted as the bookmakers' favourite and Rees-Mogg being given 50/1 odds. A day after the election on 9 June, an online petition, titled Ready for Rees-Mogg, was set up urging Rees-Mogg to run for leader of the Conservative Party. Hoping to mirror the success of pro-Corbyn activist group Momentum, a hashtag of Moggmentum was created. By 8 July 2017, the campaign had attracted over 13,000 signatures and raised £2,000 in donations with leadership odds being cut to 16/1, making him second favourite behind David Davis. On 14 August, co-founder of Ready for Rees-Mogg Sam Frost announced the petition had gathered 22,000 registered supporters, 700 volunteers and £7,000 in donations, despite Rees-Mogg having said a day earlier that such speculation was "part of media's silly season" and that "no-body serious" believed he was a candidate. On 5 September 2017, a poll conducted by ConservativeHome put Rees-Mogg as the favourite for next leader, with 23% of the votes based on 1,309 people surveyed.
Rees-Mogg was elected chair of the European Research Group, a Eurosceptic pressure group within the Conservative Party, in January 2018. A report in The Independent suggested that this position provided him with the immediate support of around 50 Conservative MPs, a sufficient number to trigger a leadership contest. Since then, Rees-Mogg directly criticised the leadership of May and chancellor Philip Hammond, fuelling more rumours that he was planning to stand for the leadership but reiterated he had no intention of doing so. In February, a speech that Rees-Mogg was giving at the University of the West of England was disrupted when left wing protesters accused him of being a racist and a bigot; violence broke out between the protesters and Mogg's supporters.
A supporter of "hard Brexit" (although he prefers the term "clean Brexit"), Rees-Mogg was highly critical of the government's handling of the Brexit negotiations, in particular Theresa May's "Chequers deal", calling it "staying in the EU without a vote":
The prime minister needs to look at what she herself has said, the promises she has made, the commitments of the last election, and see if they square with Chequers—and in my view they do not. If she sticks with Chequers, she will find she has a block of votes against her in the House of Commons. ... Of course the Eurosceptics in parliament are not in a majority on all issues, but we will inevitably be in a majority on some of them and that will make the legislation extraordinarily difficult if it is based on Chequers.
He supported a "Canada-plus" deal as a compromise; this would allow for tariff-free trade, without the UK remaining in the single market or the customs union.
In 2018, as part of a Sunday Times investigation into online abuse following controversial comments made by Boris Johnson regarding the niqab and media attention regarding alleged Islamophobia in the Conservative Party, it was reported that a number of Facebook groups supportive of Rees-Mogg and Johnson (some of which included Conservative councillors and officials) were leaving "widespread" Islamophobic and racist comments on Johnson's Facebook page. In response, Rees-Mogg said he was supporting a private member's bill put forward by Labour MP Lucy Powell to regulate social media, and added "people who have these types of views should take no solace in using [Johnson's] comments as an excuse to take this approach". Rees-Mogg defended Johnson against accusations of Islamophobia and criticised the party for initiating disciplinary action against Johnson – in order, Rees-Mogg said, to weaken Johnson politically – calling it a "low-grade abuse of power" as well as a "show trial" and a "witch hunt".
On 15 November 2018, Rees-Mogg implied that he might submit a letter of no confidence in Theresa May over her draft Brexit proposal. Later that day, he submitted such a letter to Sir Graham Brady, the chairman of the 1922 Committee of backbench Conservative MPs and told reporters "What Theresa May says and does no longer match" but added, "this is nothing to do with personal ambition". Following May's announcement that she would call off the House of Commons vote on her Brexit deal due to widespread dislike of the deal, Rees-Mogg made a statement saying: "What has two years of Theresa May doing Brexit amounted to? An undeliverable deal Parliament would roundly reject, if the prime minister has the gumption to allow it to go before the House of Commons. This is not governing, it risks putting Jeremy Corbyn into government by failing to deliver Brexit. We cannot continue like this. The prime minister must either govern or quit." In November 2018, Rees-Mogg suggested the party elect Boris Johnson as its new leader.
Rees-Mogg was described as the leading figure within the unsuccessful effort for a vote of no confidence in Theresa May as party leader by the parliamentary Conservative Party on 12 December. Despite losing the vote, Rees-Mogg continued his calls for May to resign as leader the following day, stating that the Prime Minister had "clearly lost the support of the back benches of the Conservative Party". Rees-Mogg received criticism for his role in this effort from a fellow Conservative MP Tobias Ellwood, who called his actions "destructive", "divisive" and "selfish". On 18 December, Rees-Mogg said: "Under Tory party rules the prime minister won, that is a mandate for the next year. I therefore fully support her, I lost the vote last week." He later voted against the Labour Party's motion of no confidence on 16 January 2019, having stated earlier that day on Politics Live that he would support the Prime Minister.
Rees-Mogg said on 22 February 2019 that he opposed Home Secretary Sajid Javid's decision to revoke the UK citizenship of Shamima Begum, one of the Bethnal Green trio, as she was eligible for Bangladeshi citizenship. On his Friday night show on LBC, he stated that he thought that "there is a fundamental equality in British citizens and if you can't take [his] passport away, then you shouldn't be able to take it away from anybody else" and argued that "Why on earth should Bangladeshis pick up a problem that's essentially our problem. We're trying to put our litter in our neighbour's garden."