Transcript for 'No one is above the law': Nancy Pelosi announces official impeachment inquiry
Here is ABC's David Wright. Reporter: Once again, storm clouds are gathering over the trump presidency this time with a whole new intensity. I'm announcing the house of representatives is moving forward with an official impeachment inquiry. Reporter: House speaker Nancy Pelosi had, in the past, expressed her reluctance to pursue an impeachment inquiry. Not anymore. The actions taken to date by the president have seriously violated the constitution. The actions of the trump presidency revealed dishonorable fact of the president's betrayal of his oath of office, betrayal of our national security, and betrayal of the integrity of our elections. Reporter: Tonight, the president insists he has nothing to fear. They all say that's a positive for me from the election. You could also say, who needs it? It's bad for the country. Reporter: The trump campaign says this will only "Embolden and energize president trump's supporters." And the white house accuses the Democrats of trying "To weaponize politics when they should be working for their constituents." Everybody knows it's just a Democrat witch hunt. Here we go again. Reporter: The issue, not Russia, this time. But Ukraine. And whether trump sought to enlist a foreign government's help to dig up dirt on a political rival, former vice president Joe Biden and his son hunter. The allegation here, if the facts bear it out, is very serious indeed that he was using the powers of the presidency to pressure a foreign government to essentially do harm to his political rival Joe Biden. Reporter: The key moment, a phone call that took place one day after special counsel Robert Mueller finally testified before congress. He swept swept into power on a promise to fight corruption and president trump had an idea just where to start. The conversation I had was largely congratulatory, was largely corruption, largely the fact that we don't want our people, like vice president Biden has done, creating to the corruption already in the Ukraine. Reporter: President trump admits he brought Biden up during the call but insists he didn't pressure anybody. There was no pressure. That was not pressure. I know when I give pressure. Reporter: But a whistle blower from the intelligence community says others. According to "The Wall Street journal" trump brought up Biden eight times during the call, something the president disputes. Days before the call, the U.S. Abruptly cut off $400 million in military aid to Ukraine on orders from the president. There was never any quid pro quo. Reporter: President trump has offered various reasons for withholding that money. 24 hours ago he said it was because he wanted Ukraine to deal with construction. Why would you give money to a country that you think is corrupt? Reporter: Today trump suggested he was trying to save the U.S. Taxpayers some money. I want other countries to put up money. I think it's unfair that we put up the money. It's unlikely it's going to be absolutely black and white. Here, you can't get this $400 million unless you go after Joe Biden and show me everything you get. So it will be interpretation of whatever evidence they find. Reporter: But the justice department and director of national intelligence have so far refused to hand over the whistle blower's report. The intelligence community inspector general, who was appointed by president trump determined that the complaint is both of urgent concern and credible. Reporter: The tug-of-war over the whistle blower's report is what pushed reluctant Democrats over the top. There are now at least 181 members, more than two-thirds of all house Democrats who say now is the time to impeach. The thinking with so many of these members is, if this president is allowed to negotiate with foreign leaders to investigate political opponents, there will be nothing that tethers him to the norms of civil society. The attitude behind the democratic caucus at this point has been that the president has been lawless, and he's been lawless for two years now. So where does former vice president Biden fit into all this? Vice president Joe Biden was tasked with helping after the revolution. At the same time, months after Biden delivers a robust speech, his son hunter Biden takes a position on a board of an oil and natural gas company in Ukraine under investigation. Reporter: ABC's Tom llamas recently traveled to Ukraine to investigate the Biden story. What we found were actually two stories. Joe Biden supporters said what hunter did was wrong. Reporter: Giuliani reported in a story. You asked Ukraine to look into Joe Biden. Of course I did. You just said you didn't? We have two alleged wrongs here. One is the idea of cashing in on the presidency. The other is the question Offen enlisting a foreign government do dig up dirt on your rival. He's trying to divert all the attention back to Joe Biden. The problem for Democrats is there is smoke there. Reporter: Today Biden pushed back. I knew when I decided to run this president would attack me and anyone else he thought would be a threat to his winning again. And even though every reputable publication has looked at the charge that has been made against me, they've found it baseless and untrue and without merit, that's not about to stop him. Reporter: What's next? Well, president trump now says tomorrow the white house will release the complete, unredacted transcript of that July phone call with Ukrainian president Zelinsky. He's also scheduled to meet Zelinsky face-to-face at the on Thursday, the director of national intelligence is due to appear at an open hearing on capitol hill, and Democrat Adam Schiff tweeted that the whistle blower wants to testify as well. Mitch Mcconnell slammed Pelosi's decision, saying in a statement it simply confirms that house Democrats priority is not making life better for the American people but their nearly 3-year-old fixation on impeachment. Meanwhile, both sides are looking down the road to next year's presidential election. Whatever the best-laid plans of candidates and campaigns, all of that is going to fade-in the midst of an all-encompassing Washington story. Reporter: Even if the Democrats in congress gather enough votes to impeach president trump, it's unlikely they'd have the votes in the senate to remove him from office. We have never impeached and removed a president from office. And so from the speaker of the house who had resisted impeachment plans, this is a historic day. Reporter: Meanwhile, what was already said to be a bitter election battle now has a tricky new front. I'm David Wright for "Nightline" in New York.
This transcript has been automatically generated and may not be 100% accurate.