Arguably he is the President of the United States and should therefore hold the most power, but sometimes the legislative body needs to step in when they believe something is going wrong. On the 25 of July 2019,Trump has been accused of taking a call with the Ukrainian president where he asked him to help find any information regarding Joe Biden and Hunter Biden, his son. Hunter Biden has a business in which Trump believes there has been some strange business deals.
However,in American legislation, it is illegal to ask the aid of foreign powers to help win the presidential election. As Joe Biden is running against President Trump in the 2020elections, the legislative body found this as being sufficient information to charge Trump with breaking the law. This can also tie in with the theory that the 2016 election was also rigged in a similar manner.
This is why the impeachment inquiry began.
However, Trump cannot be removed from presidential power unless a significant proportion of his Republican senators voted against him. Currently this seems very difficult as the majority of the senate support him. Only two Presidents have had to face impeachment in the past- Bill Clinton and Andrew Johnson. Therefore, we will just have to wait and see the how the public hearings pan out.
Trump has always had divided support in the US but as the impeachment inquiry plays out, it will be interesting to see if his chances in the next election will be harmed.