Aides to Pompeo have defended his plans, saying he is acting in his personal capacity and asserting that no government resources were used. They’ve also reportedly said government lawyers signed off on his remarks, though they haven’t shared the legal analysis.

Democrats aren’t buying these assertions. They say it’s impossible to separate Pompeo’s appearance from his job. They also note that Pompeo recorded the remarks in Jerusalem while on official travel, meaning that at the very least he used government resources to get there.

The key question, Democrats say, is whether the appearance violates the State Department’s own guidelines — including ones approved by Pompeo — or laws, such as the Hatch Act, that govern political activity by government officials.

“The Trump administration and Secretary Pompeo have shown a gross disregard not only of basic ethics, but also a blatant willingness to violate federal law for political gain. Congress has a responsibility to stand up for the rule of law and hold them accountable for this corrupt behavior,” said Rep. Joaquin Castro (D-Texas), chairman of the oversight subcommittee, in a statement.

Castro informed the State Department of his panel’s investigation in a letter to Deputy Secretary of State Stephen Biegun. News of the probe was first reported by the Daily Beast.

In February, Biegun wrote an email to department staffers urging them to look at a set of updated legal memos, approved by Pompeo, that had detailed guidelines on if and when State Department employees could engage in partisan activities.

One of the memos, whose existence was first reported by POLITICO, includes the following instruction: “Senate-confirmed Presidential appointees may not even attend a political party convention or convention-related event.”

There do not appear to be exceptions made for Pompeo, a Senate-confirmed presidential appointee. And in general, the restrictions are even more strict for U.S. diplomats when they are overseas.

Just last month, Pompeo sent a memo to all diplomatic and consular posts reminding State Department employees of the strict policies that restrict political activities on their part. The note referred to the same legal memos that had been shared earlier with the department by Biegun.

It’s not clear if Pompeo ordered the policy changed in more recent days to accommodate him. The State Department did not immediately respond to requests for comment on Tuesday. Some outside lawyers, however, noted that the State Department policy doesn’t have the full force of law, and that it can be changed.

State Department employees reached by POLITICO expressed deep disappointment in Pompeo, though many were not surprised given that he’s gone further than most secretaries of State in flirting with partisanship.

For instance, he has appeared in the past at events that closely align with the Republican Party, such as the Conservative Political Action Conference. He also devotes a major portion of his media time to conservative news outlets.

He also has been accused of being responsive to requests from Republicans for documents, but not to Democrats’ requests. This pattern suggests the State Department may not meet the September deadlines Castro gave Biegun to answer questions and produce records.

Democrats also are investigating Pompeo’s role in the firing earlier this year of the State Department inspector general, whose office is investigating whether Pompeo and his wife, Susan, improperly used State Department resources.

Still, the Republican convention appearance goes further than past seemingly partisan moves by Pompeo, State Department employees said.

“It’s a total sham,” one staffer said. “There is no way that the U.S. government is not funding or supporting this speech in some capacity.”

Left-leaning pro-Israel organizations have also decried Pompeo’s decision to tape his message from Jerusalem. They say the move further turns the U.S.-relationship with Israel into a Republican cause as opposed to a bipartisan one. Some critics also question whether Pompeo’s conservative Christian views are driving the decision to use Jerusalem as a backdrop.

Despite the criticism, speaking at the GOP convention offers Pompeo some political benefits.

The former Kansas congressman has allied himself closely to President Donald Trump and would likely need the support of the president’s base should he, as expected, run for office in the future.

Pompeo also takes great pains to stay in Trump’s good graces, even if it means upsetting his staffers at the State Department, and speaking at the convention could be just another way to show that he puts the president’s needs first.