Hartford — Calling President Donald Trump's pressuring of Ukrainian President Volodymyr Zelensky to investigate Joe Biden's son "the most serious moment of the Trump administration to date," U.S. Sen. Chris Murphy said he will give "serious thought" over the next couple of days to impeachment.

"No president of the United States can use his office, can use the national security apparatus of this country to try to interfere in an election to try to destroy his political opponents," the Connecticut Democrat said at a news conference Monday in Hartford.

Trump, who's admitted to asking Zelensky to investigate Biden's son, Hunter, who was a board member of a private Ukrainian gas company, Burisma Group, defended the conversation on Monday, saying he didn't threaten to withhold $250 million in U.S. military aid if Zelensky did not initiate the probe. Amid pressure from Congress, the Trump administration agreed to release the aid in mid-September.

Trump's comments come on the heels of a whistleblower complaint made by a member of the intelligence community, which reportedly involves the president's conversation with a foreign leader, deemed to be "credible" and of "urgent concern" by the intelligence community's inspector general.

The acting director of national intelligence, Joseph Maguire, who has refused to share the complaint with Congress, as required by law, has agreed to testify in open session Thursday before the House Intelligence Committee about the complaint.

"The law commands the administration to send serious whistleblower complaints to Congress, and the executive branch needs to do that ASAP," Murphy said Monday.

The Constitution gives the U.S. House "the sole power of impeachment." The Senate ultimately is the juror, Murphy said. Still, some of Murphy's colleagues in the Republican-controlled Senate have been calling on House lawmakers to start impeachment proceedings.

U.S. Rep. Jim Himes, D-4th District, was among the first members of Congress, to publicly support impeachment and is the only member of Connecticut's congressional delegation to do so thus far.

Murphy said it doesn't matter whether there was a "quid pro quo." Even if Trump did not explicitly say to Zelensky that he would withhold the aid, "there is an implicit threat in every single demand that a United States president makes of a foreign power, especially a country like Ukraine that is so dependent on the United States," he said.

"Every time the American president asks a foreign country to do something, that foreign country knows that if they don't do it, there are likely going to be consequences," he added.

Even if Trump had just asked for an investigation, and not mentioned the aid, "that would be unacceptable in and of itself," Murphy said.

"We just spent a year trying to figure out if the president invited the Russians to interfere in the 2016 elections. Had the Mueller investigation uncovered a phone call between Donald Trump and Vladimir Putin in which Donald Trump asked Vladimir Putin eight different times to interfere in the 2016 election that would be the smoking gun that Republicans and Democrats have been waiting for," he said.

"For me it's always been a balancing act of reasons to withhold a recommendation for impeachment against the gravity of the actions by the administration and certainly this weekend's disclosures make the gravity of abuse by the White House even more serious and that will factor into the decision that I make in terms of my public position on this matter," Murphy said.

Murphy said the effort by the Trump administration to pressure Ukraine goes beyond the phone call between Trump and Zelensky on July 25.

A member of the Senate Foreign Relations Committee, Murphy said he first heard in the spring that Zelensky "was deeply concerned and confused" about the president's personal lawyer, Rudy Giuliani, traveling to Ukraine and allegedly pressuring leaders there to open an investigation into the business dealings of Hunter Biden.

"Fairly immediately" after hearing about Zelensky's concern, Murphy sent a letter to U.S. Sen. Jim Risch, R-Idaho, chairman of the committee, urging him to "submit an official Senate query to the Trump administration asking why a private citizen is traveling to Ukraine to work with a foreign government in an effort related to President Trump's re-election campaign."

Risch declined to open a query. Murphy wrote to Risch again last week, amid reports of the whistleblower complaint, urging him to investigate the matter.

Murphy met with Zelensky earlier this month during a trip to Ukraine with U.S. Sen. Ron Johnson, R-Wis., who opened the meeting by raising concerns about the aid being withheld and the effect it would have on Ukrainian security. Russian-backed separatists, who hold a substantial part of eastern Ukraine, have been battling with the Ukrainian military for more than five years.

Murphy said Zelensky said during their meeting that he had "no interest" in getting involved in the 2020 election and that "he understood the damage it would do to the U.S.-Ukraine relationship if he did get involved."

The revelations surrounding Trump's conversation with Zelensky did not lead any more of Connecticut's delegates to call for impeachment.

In a statement issued Monday, U.S. Rep. Joe Courtney, D-2nd District, who represents southeastern Connecticut, said the president and Maguire, the acting director of national intelligence, "have two simple choices when he comes before Congress later this week: follow the law and transmit the complaint to the bipartisan intelligence committees, or flagrantly disobey the law and force the Congress to act to uphold the rule of law."

Courtney, during a meeting with The Day's Editorial Board in early August, said he supported the gathering of more evidence before coming to a conclusion on impeachment.

Reps. Rosa DeLauro, John Larson, and Jahana Hayes, all Democrats, have not backed impeachment. DeLauro has said the Democrats should avoid impeaching Trump and instead oust him during the 2020 election. Larson and Hayes have said Congress needs to gather more information. Hayes has said she thinks the best way to "remove" Trump is by voting him out.

j.bergman@theday.com