For months, House of Representatives Speaker Nancy Pelosi has resisted the full-throated cries of some members of her caucus to impeach President Donald Trump.

Pelosi saw the futility of launching an impeachment investigation for strictly partisan reasons. Such a move might make it through the Democratic-controlled House, but would never get through the Republican-controlled Senate, and would probably bolster the political support for the president.

But this week Pelosi launched an impeachment inquiry against President Donald Trump. She apparently senses controversy regarding U.S. relations with Ukraine might provide an opening against the president, or that it constitutes a clear-cut, impeachable offense.

House Democrats plan to investigate contacts between Trump and Ukrainian President Volodymyr Zelenskiy. They are focusing on a July 25 phone conversation in which Trump discussed a situation involving former Vice President Joe Biden, currently the frontrunner for the Democratic nomination for president.

In his conversation with Zelenskiy, Trump referenced an event in March 2016, when Biden, representing the United States, traveled to Ukraine and told the country’s leaders then they would not receive $1 billion in promised U.S. aid unless the country’s chief prosecutor was fired.

Biden admits doing just that. But he was acting on the instructions of the Obama administration, and they insist the Ukrainian prosecutor was corrupt.

It happens that the prosecutor was looking into allegations of corruption involving Biden’s son, Hunter. Biden’s action brought that probe to an end.

At the time, Hunter Biden was being paid as much as $50,000 a month to serve as a board member for Ukraine’s largest private gas company, Burisma. The firm clearly was attempting to buy influence with then-President Barack Obama’s administration. Sensing the political fallout possible from the arrangement, Hunter Biden resigned his post this spring — as his father was preparing to run for president against Trump.

Trump’s phone call was an attempt to get the Ukrainian investigation involving both Bidens resumed. Trump had held up millions in U.S. military aid to the Ukraine days before the phone call in an apparent attempt to increase the pressure on Zelenskiy. Pelosi and other Democrat leaders maintain the president’s action was an improper use of U.S. clout for political reasons.

Pelosi’s party may have opened a Pandora’s box. Any attempt to cover up Biden’s role in the affair will be seen through by voters. If the House investigation is to be viewed as anything other than what Trump says it is — a witch hunt — lawmakers cannot give Biden a free ride.