The impeachment train might be getting ready to leave the station.

Seven House Democrats who represent competitive districts are pushing to begin impeachment proceedings against President Trump, if reporting turns out to be true that Trump pressured Ukraine to investigateformer Vice President Joe Biden.

"For all seven of us, the idea that a sitting president would use security assistance from the United States to pressure and potentially extort the president of another country into giving him dirt on a political opponent is just beyond the pale," Rep. Elissa Slotkin of Michigan told NPR's Morning EditionTuesday.

Slotkin, and Reps. Gil Cisneros of California, Jason Crow of Colorado, Chrissy Houlahan of Pennsylvania, Elaine Luria of Virginia, Mikie Sherrill of New Jersey, Abigail Spanberger of Virginia all have national security backgrounds and penned an op-ed that published Monday night in the Washington Postcalling for impeachment despite the centrist political leanings of their House districts. None except for Crow had publicly backed impeachment publicly before.

Overall, on average, just 1 percentage point separated Trump and Democrat Hillary Clinton in those districts. They are all freshmen, who replaced Republicans in the 2018 wave election that ushered in Democratic control of the U.S. House.

They are exactly the kind of freshmen Democrats House Speaker Nancy Pelosi, D-Calif., has been trying to protect in being cautious about going forth with impeachment proceedings. But if the people she is trying to give cover to pull the cover off, impeachment proceedings look far more likely.

Slotkin acknowledged the potential political risk on Morning Edition, but said, "It doesn't matter." She noted that voters in her district are divided on whether to impeach Trump.

"I get pulled over in the supermarket by people talking about it and saying, go ahead and do it," she said, "and I've been pulled over by just as many people saying, please don't do it."

But, she added, "No matter whether you're a Democrat or an independent or a Republican, the idea that a sitting president would attempt to leverage dirt on a political opponent from a foreign leader is just beyond the pale, it's a game-changer, it's something different, and we have to acknowledge it as thus."

Pelosi is meeting with her Democratic caucus Tuesday afternoon. A slim majority of House Democrats were already in favor of beginning impeachment proceedingsbefore the Ukraine call incident, which stemmed from a whistleblower complaint by a member of the intelligence community.

Even more Democrats have come out in favor since, and that number could rise after reporting late Monday night from multiple outletsthat the president ordered $400 million in potential aid to Ukraine to be held back before the phone call with the Ukrainian leader.

Trump reportedly pressured the Ukrainian leader to investigate Biden and his son Hunter, who served on the board of a Ukrainian gas company, in a July 25 phone call. Trump denied that allegation Tuesday morning before a speech before the United Nations General Assembly.

"I think it's ridiculous," Trump said. "It's a witch hunt." He noted, despite polling evidence to the contrary, that he is "leading in the polls" and the "only way to stop me is through impeachment." He said this has "never happened before."

He added that he assumes people will see a readout of the call with the Ukrainian leader and "that call was perfect," he said, asserting, "There was no pressure put on them whatsoever."

Instead, he contended the pressure was "put on by Joe Biden. What Joe Biden did for his son, that's something they should be looking at."

Trump admitted that he put a hold on money to Ukraine, but he denied it had anything to do with wanting Ukraine to investigate Biden.

Instead he did it, "because-- very important, very important, I want other countries to put up money. I think it's unfair that we put up the money but then people call me and they say, 'Oh, let it go, and I let it go.' "

The allegation, popular in conservative media circles, has been looked at. And there is no evidence the Bidens did anything wrong, something that has been fact checked by multiple news outlets.

As part of Obama administration policy in an effort agreed to by other Western alliance countries, the International Monetary Fund and the World Bank, Biden threatened to hold up funding to Ukraine unless a prosecutor alleged to have engaged in widespread corruption was ousted.

The then-vice president was sent to Ukraine with that message, and it worked. It's not something Biden tried to hide. He, in fact, touted it last year.

As NPR's Lucian Kim reported from Kyiv Tuesday morning, the allegations of corruption against the gas company, Burisma, pre-dated Hunter Biden joining its board. In fact, Kim reported, Burisma brought Hunter Biden on board in an effort to look like it was cleaning up its act. And many in Ukraine credit Joe Biden with helping in the effort to clean up corruption in the country.

"Without Hunter Biden, there would be no story here," Brian Bonner, chief editor of the Kyiv Post, said on Morning Edition. "Hunter Biden looks like he was trying to cash in."

But, Bonner added, "Biden, as vice president, did not try to kill the investigation into Burisma. In fact, one of the reasons why the prosecutor general was fired was because he obstructed one corruption case after another. He prosecuted no one for corruption; he protected corruption."

The president's personal attorney, Rudy Giuliani, has admitted to pressuring Ukrainian officials to investigate Biden and met with one official in Madrid in an encounter set up by the State Department, he told the Wall Street Journal.

Trump is trying to sell that he is the victim here, a tactic he's employed many times before to show solidarity in grievance with his political base. But this behavior, if true, is something even Republicans are saying would have been inappropriate.

The issue, however, is that the Trump administration controls much of the information that would answer many of the questions reporters and Congress are asking. The administration has not released the whistleblower complaint publicly or even to Congress, and members of his administration have resisted releasing any transcript of the call Trump had with the Ukrainian leader, citing the precedent it could set.

All of that gives a degree of political cover to Republicans. They can say, if true, it would be inappropriate, but more information is needed. And yet that information is controlled by executive branch — and if it's never released, it allows Republicans to continue not criticizing him and hope the controversy fades.

But for Democrats, even this new group of moderates, they are putting the responsibility on the administration to prove the president did nothing untoward.

"The onus is on them in the administration to prove that there wasn't some serious wrongdoing here, given that the president's lawyer said it openly," Slotkin said. "He admitted it, so the onus is on them. Otherwise, I think we know we need to move to another tact here."

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