It’s probably fair to give substantial credit to Sen. Jeff Merkley for helping bring the issue of immigrant children being separated from their parents to national attention. Two weeks ago, the Oregon Democrat was blocked from entering a detention facility in South Texas, even having police called on him, as he made a fairly simple case for being allowed entry: I’m a member of the Senate. Eventually, he was able to tour the facility.

Over the next few weeks, as attention to the subject increased, other elected officials and journalists sought — and occasionally gained — access to facilities housing immigrant children. In some cases, though, members of Congress weren’t allowed to see facilities, even ones in their own districts. On Tuesday, Sen. Bill Nelson (D-Fla.) was denied access to a facility in his state when he tried to visit along with other legislators.

“It is an affront as the senior senator of this state that an agency head would tell me that I do not have entrance into a federally funded facility where the lives and health of children are at stake,” Nelson said afterward.

That is an affront to a senator — but as it turns out, there’s probably not much Nelson could do about it.

Lou Fisher worked for Congress for 40 years as senior specialist in separation of powers with the Congressional Research Service and at the congressional law library. He’s testified more than 50 times before Congress as an expert on constitutional issues. He spoke with The Washington Post by phone Thursday morning and explained the limits of Congress’s ability to drop in on government facilities.

“I can’t think of any examples in the past where members of Congress wanting to do oversight were turned away,” Fisher said. He made clear, though, that allowing legislators into government facilities is generally a function of protocol, not an inherent power granted to Congress.

“The agency” — in this case, the Department of Health and Human Services — “has a duty to tell members when they can come,” he said. “It has to be cleared.”

On Wednesday, HuffPost published an email from HHS that outlined rules for legislators who want to visit: Certain days were being made available for visits over the short term, but individual visits would require a two-week lead time.

Fisher found that odd.

“Why would the administration do this two-week notice?” he asked. “What was the point behind that? If you’re going to let them come in two weeks, then why not one week or one day and so forth?”

In a conversation with The Post this week, Rep. Mark Pocan (D-Wis.) offered his take on that point — to guide legislators to see only certain things.

“Once you’ve scheduled something, they will try and arrange a tour,” Pocan said. “The problem is they always want to show us the medical facilities and the kitchen rather than talking to people. They often won’t let us talk to people at the facilities, other than when they select someone for us.”

He described having arrived slightly later than scheduled to tour an Immigration and Customs Enforcement facility. It was 8:30 p.m., and he and his delegation were told that mothers of children removed by the administration weren’t able to talk, because they had all gone to bed. Eventually, ICE relented.

“There still is an access problem,” Pocan said. “I think the people who are contracted to run them sometimes claim liability and other concerns, but the bottom line is we can’t actually get information had we not been able to kind of force the conversation.”

Fisher noted that congressional committees could force people to provide testimony during hearings, holding people in contempt of Congress if they refused a summons. But traveling elsewhere to conduct oversight, even with the imprimatur of a formal congressional delegation approved by the majority of an oversight committee, doesn’t have the same sort of weight.

“It’s a political issue of how much you want to offend members of an oversight committee,” Fisher said. He suggested that the administration would probably need to give a reason for denying access but that, fundamentally, denying entry to members of Congress would eventually be resolved with political pressure, not legal fights.

“We really do have to force the conversation, and it shouldn’t be,” Pocan said when we spoke. “If there was nothing to see here, so to speak, you wouldn’t be necessarily trying to hide people who are in charge of funding from being able to see that.”

That’s an argument that would probably carry some weight in a political debate.