“This is not about the president, this is about the presidency,” said Mr. Lankford, who was a longtime Baptist youth minister before he entered politics. “This is about where we are as a nation.”
This is not to say that other members of the panel aren’t engaged. The committee’s seven Democrats are certainly interested in finding out whether Russians colluded with the Trump campaign and helped to elect him.
Senator Richard M. Burr, Republican of North Carolina and the chairman of the panel, has shown an increasing zeal for pursuing the question after an uncertain start. He and Senator Mark Warner of Virginia, the committee’s ranking Democrat, have forged a solid working relationship.
Three other Republicans are also playing a role: John Cornyn of Texas, who as the No. 2 Senate Republican brings a leadership perspective to the investigation, Jim Risch of Idaho and Tom Cotton of Arkansas.
But it is notable that the other four have quietly coalesced into something of an informal working group within the Intelligence Committee, pushing the investigation forward and consulting not only with each other and Mr. Burr, but also with Mr. Warner.
“We are working very hard and we talk a lot with one another, as well,” said Ms. Collins, who said the investigation would “take as long as required.”
“This is a complex investigation, and as you pull the threads, you find that it is connected to a whole lot of other threads in this tapestry that we are not yet seeing the whole of.”
Here is a look at the four and what is driving them:
Although she is known as the Republican centrist voice in the Senate, another role she has held in Washington may be equally important in this case: senior Senate staff member.
Ms. Collins was a top Senate aide and served in other executive posts before running for office. She is experienced in both conducting and overseeing inquiries.
“I really want to know the truth no matter who is implicated, no matter where the evidence leads,” she said. As a 21-year-old in 1974, she was an intern for Representative William S. Cohen, a freshman Republican congressman from Maine who helped draw up the articles of impeachment of President Richard M. Nixon.
His colleagues say it would be a mistake to underestimate this junior member of the Senate.
Mr. Lankford showed surprising political strength in a 2014 primary fight in a special Senate election in Oklahoma after compiling a conservative record and rapidly raising his profile during two terms in the House.
He objected sharply to recent reports that the Senate inquiry was understaffed and moving at a plodding pace.
“If you make a big staff, they get less access to the real documents for intelligence that you need,” he said. “You need to keep it with high-level folks in as small a pool as possible and give them the time they need,” he said.
Very few members of Congress make it into the leadership ranks; hardly anyone makes into leadership in the House and in the Senate.
Mr. Blunt, the former House majority leader and a savvy inside player, is now the fifth-ranking Republican in the Senate. He has been adamant that Congress pursue the investigation into Russian meddling — both to find out what happened and to allow Congress and the White House to move beyond it.
“Everyone would benefit if we do this job in the right way and do it not faster than we can, but as fast we can,” he said.
Mr. Blunt has been a consistent voice that the committee must be thorough. “When we are done, we need to have talked to everybody a reasonable person would think we should talk to and have seen everything a reasonable person would think we should see,” he said.
After his failed presidential bid, he almost didn’t return to the Senate, but a change of heart has thrust him into the middle of an inquiry surrounding the election of his Republican primary race rival.
In a recent appearance on CBS’s “Face the Nation,” Mr. Rubio suggested that committee work would not just lay out for the public what the Russians did, “but how they did it and what it means for the future and what we should be doing about it.”
A proponent of a hard line with Russia, Mr. Rubio dismissed Mr. Trump’s complaint that he was the victim of a witch hunt. “We are nation of laws and we are going to follow those laws,” he said. “The president is entitled to his opinion.”
There is no doubt that political conflict will erupt as the inquiry advances. These four senators will be crucial in determining whether it stays on track.