They are the most closely watched senators of President Trump’s impeachment trial — the moderate Republicans who might vote with Democrats to call new witnesses and subpoena unseen documents. But, a week into the trial, they are also oddly isolated.

Trump has kept his distance after White House advisers warned him that outreach would not help his cause. Democrats say there is no serious effort to privately lobby the Republicans, with the party relying instead on public opinion and the House prosecutors to squeeze them into breaking ranks.

The group of senators, some facing tough elections in November, could be decisive next week, defying GOP leaders and voting with Democrats for the witnesses and evidence that the Trump administration has repeatedly denied Congress. Such a move could extend the trial indefinitely, shed new light on Trump’s conduct and pose a serious threat to his presidency.

At the Senate impeachment trial of President Trump, Democrats continued to make their case for abuse of power while Republicans zeroed in on the Bidens. (The Washington Post)

Under the microscope are centrists such as Sens. Susan Collins (Maine), Lisa Murkowski (Alaska), Mitt Romney (Utah) and Lamar Alexander (Tenn.), but also Sens. Joni Ernst (Iowa), Cory Gardner (Colo.) and Martha McSally (Ariz.) and Thom Tillis (N.C.), who, like Collins, face voters this year.

An incessant operator of the telephone, Trump has not taken to calling the potential swing senators to plead his case. Advisers, who spoke on the condition of anonymity because they were not authorized to discuss the matter publicly, said they have cautioned the president that calls to members such as Collins, Murkowski and Romney would not influence their decisions. So Trump has left them alone — returning to a tack he used during the successful fight to confirm Supreme Court Justice Brett M. Kavanaugh in 2018 — while regularly checking in with Senate Majority Leader Mitch McConnell (R-Ky.), who has marshaled Trump’s trial defense.

Democrats have not made it easy. They used a series of procedural votes Tuesday night and early Wednesday morning to box in GOP leaders, potentially putting their goals of protecting Trump and ensuring their majority at odds by forcing the moderates to cast votes against summoning documents and witnesses.

What has so far been absent, multiple senators and aides said, is any sort of bipartisan dialogue aimed at brokering an agreement to secure testimony from key witnesses such as former national security adviser John Bolton and acting White House chief of staff Mick Mulvaney.

“They’re going to make a decision based on what they think,” said Sen. Debbie Stabenow (D-Mich.). “People have to decide: country or party.”

The tenor of the Democratic pressure campaign has prompted some Trump loyalists to view the impeachment trial as less about the president and more about Democrats’ efforts to seize power on Capitol Hill.

“The goal of this entire process is not to remove the president from office, it’s simply to remove certain Republican senators,” said Sen. Tim Scott (R-S.C.). “This is absolutely an opportunity for them to try and overtake the Senate.”

The most vulnerable senators have adopted different approaches to the impeachment scrutiny. Collins has made a conspicuous display of her deliberations, taking copious notes during the trial and issuing public statements lending qualified support to the notion of witnesses. Gardner has stayed mostly silent, while Ernst, McSally and Tillis have been more eager to embrace Trump.

All, however, voted in near-lockstep with McConnell against the Democratic amendments for witnesses and evidence Tuesday and Wednesday — only Collins broke ranks, voting for one of 11 amendments to McConnell’s rules.

Asked about the Democratic pressure Thursday, Ernst was defiant.

“Bring it on, folks,” she said. “We’ve got a strong economy. Unemployment in Iowa is at an all-time low. We’re fully employed in this state. . . . I mean, you name it, we have had a number of successes here. And if they want to argue [impeachment] with me, bring it on.”

The pressure will build: More votes are expected next week, after the House managers and Trump’s defense team present their arguments. Some Democratic senators held out the possibility of bipartisan talks about summoning additional evidence at that point but suggested it would depend on whether the outside pressure campaign succeeded.

Democrats have public opinion on their side. Two polls released Wednesday found significant majorities of Americans believe the trial should include new evidence.

A Monmouth University poll found that 57 percent of respondents believed the House impeachment managers should be able to present new evidence in the Senate trial, while a CNN poll found that 69 percent favor hearing from witnesses who did not testify during the House impeachment inquiry.

“The American people seem to agree with us,” Senate Minority Leader Charles E. Schumer (D-N.Y.) said Thursday. “Am I certain that we’ll get those four Republicans? Absolutely not.”

Democratic campaign groups have ratcheted up the pressure — particularly on Collins, who called for witnesses in the 1999 impeachment trial of President Bill Clinton — but what they have not done so far is put significant money behind those messages.

Instead, multiple Democratic campaign officials said, they plan on continuing a tried-and-true strategy of focusing on health care and other proven messages while impeachment pressure mounts through home-state media coverage.

“Let’s be clear about one thing: Trump’s position on cutting Social Security and Medicare will be featured in far more ads this year than the fact that he has been impeached,” read a memo issued Wednesday by Priorities USA, a major Democratic super PAC that is expected to spend millions in key states.

But the Democratic officials, who spoke on the condition of anonymity because they were not authorized to comment publicly, said the vulnerable senators’ impeachment votes would make it difficult for them to distance themselves from Trump and McConnell — who are unpopular in key Senate battleground states.

Some Democratic money is pushing impeachment messages: Presidential candidate Mike Bloomberg, for instance, is planning to run TV ads in key Senate states declaring, “If they won’t do their jobs, this November, you and I will.”

Two groups run by disaffected Republicans, the Lincoln Project and Republicans for the Rule of Law, have launched relatively modest ad campaigns targeting the GOP moderates.

Trump, meanwhile, has suppressed his instincts to work the phones. Besides leaving the GOP swing votes off his call list, he also has avoided contacting potential Democratic votes for acquittal, Sens. Joe Manchin III (W.Va.) and Doug Jones (Ala.), aides say.

While he has agitated at times to have the case dismissed, pushing advisers on a recent flight to Wisconsin to get rid of it, he has not tried to force his hand with senators. He has instead been more focused on White House communications and political messaging, said aides, who spoke on the condition of anonymity to talk frankly.

The strategy instead, senators say, is to turn senators against the House managers arguing the case. Trump has told his legal team to attack the lead impeachment managers — Intelligence Committee Chairman Adam B. Schiff (D-Calif.) and Judiciary Committee Chairman Jerrold Nadler (D-N.Y.) — as much as they can, according to White House aides familiar with the conversations.

Some of his formal and informal political advisers have cautioned him to let the Democrats talk — and that the American public, and senators, would tire of the impeachment managers.

The strategy appeared to be working on Murkowski, who gave a brief but cutting review of the managers’ case Thursday. “It was long. It was very repetitive,” she said. Murkowski voted with McConnell against witnesses and evidence. It was unclear how she will vote next week.

But inside the Senate chamber, there have been some moments when Democrats have appeared to make an impression.

Around 9:30 p.m. Wednesday, Schiff reviewed the case of former U.S. ambassador to Ukraine Marie Yovanovitch and the smear campaign undertaken by Trump’s allies to “get her out of the way.”

In telling the story, Schiff repeatedly directed his gaze toward Collins and Murkowski, seated side-by-side behind Republican leaders. They appeared engrossed, and their expressions became more troubled as Schiff continued.

“I think this is some form of cosmic justice that this ambassador that was so ruthlessly smeared is now a hero for her courage,” he said, praising many of the executive branch officials who came forward to testify over White House opposition.

Schiff also reminded Republicans that new evidence could emerge later this year. That could be more consequential for Democrats looking toward the November elections.

“More emails are going to come out. More witnesses are going to come forward. They’re going to have more relevant information to share,” Schiff said. “And the only question is, do you want to hear it now? Do you want to know the full truth?”

Karoun Demirjian, Emily Guskin, Seung Min Kim and Elise Viebeck contributed to this report.