Senate Foreign Relations Committee ranking member Bob Menendez Robert (Bob) MenendezWatchdog confirms State Dept. canceled award for journalist who criticized Trump Kasie Hunt to host lead-in show for MSNBC's 'Morning Joe' Senators ask for removal of tariffs on EU food, wine, spirits: report MORE (D-N.J.) on Tuesday questioned Vice President Pence and Energy Secretary Rick Perry Rick PerryEnergy secretary questions consensus that humans cause climate change OVERNIGHT ENERGY: Democrats push resolution to battle climate change, sluggish economy and racial injustice | Senators reach compromise on greenhouse gas amendment stalling energy bill | Trump courts Florida voters with offshore drilling moratorium OVERNIGHT ENERGY: Trump signs major conservation bill into law | Senate votes to confirm Energy's No. 2 official | Trump Jr. expresses opposition to Pebble Mine project MORE regarding their travel to Ukraine as the White House faces scrutiny for its dealings with the country.

The questions in letters Menendez sent to Pence and Perry centered around Pence’s canceled trip to Ukrainian President Volodymyr Zelensky's inauguration and Perry’s trip in his place.

"Given your role as the leader of the official United States delegation to the inauguration, and the White House’s recent release of evidence that President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE requested that Ukraine’s government investigate his domestic political opponent, your insight into the U.S. delegation’s trip is of particular interest to the Senate Foreign Relations Committee," Menendez wrote in his letter to Perry.

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Menendez asked the Energy secretary about who asked him to lead the delegation and why, as well as whether he was asked to convey "President Trump’s desire for assistance in investigating one of his political opponents or their family members, or unsubstantiated theories related to Ukraine’s involvement in the 2016 U.S. election."

The Democrat asked Pence why his trip was canceled and also questioned him on his travel to the country in September.

"You met with President Zelensky in Warsaw, Poland. When asked by a reporter on that trip whether you could ‘assure Ukraine that the hold-up of that money has absolutely nothing to do with efforts, including by Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE, to try to dig up dirt on the Biden family,’ you did not answer the question,” he wrote to Pence.

“As the summary of the call with President Zelensky demonstrates, President Trump requested a foreign government help investigate his domestic political opponent. While I hope that he was alone in making such an inappropriate request, your statements regarding your discussions with Ukraine officials raise questions whether you may have helped carry that message,” he added.

Menendez requested that the administration officials answer his questions by this Friday.

The inauguration trip is mentioned in the recently released whistleblower complaint in which Trump is accused of pushing Zelensky to investigate Democratic presidential candidate Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE for Trump's benefit. The president has denied wrongdoing.

"On or around 14 May, the President instructed Vice President Pence to cancel his planned travel to Ukraine to attend President Zelenskyy' s inauguration on 20 May," the complaint said, citing U.S. officials. "Secretary of Energy Rick Perry led the delegation instead."

"According to these officials, it was also 'made clear; to them that the President did not want to meet with Mr. Zelenskyy until he saw how Zelenskyy 'chose to act; in office. I do not know how this guidance was communicated, or by whom. I also do not know whether this action was connected with the broader understanding, described in the unclassified letter, that a meeting or phone call between the President and President Zelenskyy would depend on whether Zelenskyy showed willingness to 'play ball,' " it continued.