Twenty Republicans would have to abandon Trump in the Senate to convict him. So far, some Republican politicians have supported Trump's strategy of denials and claims, which seeks to cast the inquiry as a partisan push.
Trump will be investigated over a call on July 25 to his counterpart in Ukraine, in which Trump — according to a version released by the White House — asked for "favour". Trump wanted a corruption probe into his 2020 presidential rival former Vice-President Joe Biden and his son. To prod the Ukrainians, the US President reminded President Volodymyr Zelenskiy of US aid for the European country — appearing to twist US foreign policy in service of Trump's self-interest.
Trump has confirmed that he held up delivery of military aid before the call, saying he aimed to encourage Europeans to contribute more. Ukraine has been involved in a conflict with its neighbour Russia and the US military aid is vital.
According to a whistleblower complaint, White House officials intervened to "lock down" the transcript of the call. It was stored in a special secure system. Other call transcripts have been stored in the same way.
There could be a lot still to emerge. For instance, was there any link to the fact that just days after the call, the director of national intelligence Dan Coats and his deputy Sue Gordon were forced out?
CNN reports that the White House restricted access to Trump's calls with Russian President Vladimir Putin and Saudi Arabia's Crown Prince Mohammed bin Salman.
The Washington Post reported that in May 2017, "Trump told two senior Russian officials in an Oval Office meeting that he was unconcerned about Moscow's interference in the 2016 election because the US did the same in other countries… That prompted alarmed White House officials to limit access to the remarks to an unusually small number of people." For Trump this will be a period of sustained, inescapable pressure.
Trump himself hasn't changed. He's the president he always appeared likely to be. But people around him may start seeing his presidency differently.