They were billed as the most important midterm elections in decades, as Americans went to the polls on Tuesday to deliver their first political verdict on Donald Trump's presidency. But 24 hours later they seemed a distant memory, as Washington was plunged back into the political drama that has been the hallmark of the Trump era.

On Wednesday morning, as Americans were still digesting the results of the United States' congressional and gubernatorial races, the White House summoned reporters for a press conference. I was among the dozens of journalists who went along for what we expected to be a regular briefing.

Presidents typically talk to the media after election day. Most express contrition for their side's losses – midterms often go against the party that holds the White House – and promise to work in a new spirit of bipartisanship. George W Bush admitted in 2006 that voters had given him a thumping, and in 2010 Barack Obama apologised for his party's "shellacking". Not Donald Trump.

Wednesday's 90-minute press conference functioned as an extended victory lap for the mercurial president. Tuesday had been a "big, big day, an incredible day", for Republicans, he said, brushing aside the Democratic Party's victory in the House of Representatives. He suggested that Republican politicians who lost their seats had suffered because they hadn't endorsed him. He read out their names one by one. "Too bad, Mike," he said of Mike Coffman, who lost his seat in a part of Colorado where Trump is unpopular. "Mia Love gave me no love, and she lost. Too bad. Sorry about that, Mia," he said of the Utah Republican.

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At least one party colleague took umbrage as he watched the press conference. Ryan Costello, a retiring Pennsylvania congressman, tweeted his disgust at the president of the United States, or Potus. "To deal w harassment & filth spewed at GOP MOC's" – Grand Old Party, or Republican, members of Congress – "in tough seats every day for 2 yrs, bc of POTUS; to bite ur lip more times you'd care to; to disagree & separate from POTUS on principle & civility in ur campaign; to lose bc of POTUS & have him piss on u. Angers me to my core."

But Trump's altercation with CNN's chief White House correspondent, Jim Acosta, took the president's aggression to a new level. As he shouted at Acosta to sit down, after the journalist repeatedly tried to challenge Trump's scaremongering about the caravan of migrants walking through Mexico towards the US border, the air bristled. A physical confrontation was averted, but only just.

Miraculously, Trump calmed down – before moving to his next targets, subjecting two African-American women to his scorn. He repeatedly told April Ryan, a radio correspondent who also contributes to CNN, to sit down and accused Yamiche Alcindor of PBS NewsHour of asking a racist question.

As the meandering press conference continued, Trump’s mood swung from angry derision to strange humility as he played the role of misunderstood victim. At times he seemed more autocrat than US president, picking and choosing which journalists to talk to and which questions to answer.

The alarming contrast with other press conferences I have attended, whether with taoisigh or the likes of the former British prime minister David Cameron or the German chancellor, Angela Merkel, was a measure of how far US presidential behaviour has fallen since Trump's election.

But if Wednesday’s press conference was yet another low, it also showed that the former reality-television star remains a master media manipulator, shaping the narrative of his presidency. Trump expertly refocused coverage away from his party’s poor performance in the house and towards himself.

His tussle with Acosta was likely premeditated. Trump, after all, decided to take a question from a reporter with whom he has frequently clashed. Many believe he wanted confrontation. Clearly enjoying himself, he at one stage asked, rhetorically, “Should we keep this going for a little while?”

That Wednesday's spectacle was less a press conference and more an opportunity for Trump to play the bully boy was underlined by the fact that, having been specifically asked about his attorney general's future, he chose instead to wait an hour and then announce on Twitter that he had effectively fired Jeff Sessions. That the media briefing had been Trump's opportunity to brief the media seemed to have been lost on the president.

Despite his changing of the narrative, this week’s elections will significantly change politics in Washington for the second half of Trump’s term. The midterms have ushered in a new political reality: his White House is faced with a divided government for the first time. Their new congressional power will allow Democrats to frustrate the president’s agenda. From the proposed border wall to military funding to healthcare, the Trump administration will find it exceedingly difficult to push its proposals through Congress.

More concerning for the president is his opponents' new powers of subpoena. Richard Neal, the Massachusetts congressman who is expected to become the next chairman of the ways-and-means committee, the house's chief tax-writing body, has already said he intends to seek Trump's tax returns under a 1924 law that lets certain legislators demand the release of any American's tax details, although he anticipates a legal battle with the White House.

The expected incoming chairman of the judiciary committee, the staunch Trump critic Jerry Nadler, has pledged to preserve the integrity of Robert Mueller’s investigation into Russian interference in the 2016 presidential election, amid suggestions that the White House may seek to keep private most of the information in the special counsel’s report.

Despite Trump's early talk of a "beautiful bipartisan" future with Democrats, Nancy Pelosi, who will soon move from minority to majority leader in the House of Representatives, replacing the retiring Republican Paul Ryan, was quick to warn that Trump's firing of Sessions was unacceptable and a blatant attempt to undermine the Mueller inquiry, which could submit its report at any time.

Much focus in the coming weeks will be on how far the president tries to constrain its investigators' work via his new acting attorney general, Matthew Whitaker, before the new Congress is sworn in, in early January, and Democrats assume the majority. Whitaker, a Trump loyalist who has publicly criticised the Mueller probe, has indicated that he has no intention of following Sessions, whose chief of staff he was until Wednesday, in recusing himself from overseeing the Russia investigation. (Although any action Whitaker takes, on that or any other issue, will be illegal, according to several prominent legal scholars, including the Harvard professor of constitutional law Laurence Tribe, as the United States Senate has not confirmed his appointment.)

The clashes that will flow from the realignment of power in Washington reflect the picture of an even more politically divided United States that the midterms have drawn. Both parties can claim victory from Tuesday’s results, Democrats for seizing control of the House of Representatives after winning the all-important suburban and women’s vote, Republicans for expanding their majority in the senate – largely on the basis of support from rural voters – which will allow the GOP to continue to pack federal courts with conservative judges.

This week has lessons for both parties as the next presidential election looms. Republicans boasted that they won the key races for governor in Ohio and Florida, two battleground states that will again play an outsized role in 2020. But Democrats outperformed in Pennsylvania, Michigan and Wisconsin, whose narrow majorities for Trump in 2016 ensured his victory.

The midterms might also show the limits of campaign spending. These elections were the most expensive in history – an estimated $4.7 billion was spent, with Democratic fundraising far outstripping Republican – but this did not always translate into votes, as shown by Beto O’Rourke’s inability to flip Ted Cruz’s senate seat, in Texas, despite the huge fundraising machine behind him.

Perhaps most significantly, the midterm elections did not produce a decisive answer to a question that will shape the outcome of the next election: what kind of candidate does the Democratic Party want as it tries to dislodge Trump in 2020?

It has still not fully resolved the internal divisions left by the battle between Bernie Sanders and Hillary Clinton for the Democratic nomination in 2016, and the midterms likewise yielded no clear sense of how the party hopes to define itself. O'Rourke's fellow gubernatorial candidate Andrew Gillum, in Florida, was one of a number of other progressives who also failed to win their races, and none of this week's elections produced a standout candidate.

The midterm elections traditionally mark the unofficial start of the next presidential race. Better-known potential Democratic candidates, such as Joe Biden or even Sanders, can perhaps afford to wait a little longer, but other contenders for 2020 will need to declare their intentions as early as January.

Trump has already created a formidable re-election campaign – Trump 2020 baseball hats are a frequent sight at campaign rallies. As the United States turns its eyes to that big election battle, the next two years of his administration promise to be as eventful as the first, as the polarising president prepares to fight for a second term in the White House.