Washington (CNN) Donald Trump's team, led by Vice President-elect Mike Pence, is gently sussing out what it would take to win enough support in the Senate to fill the vacant seat on the Supreme Court -- a surprisingly tough prospect with the threat of a filibuster looming and a quick deadline set by Trump.

Behind the scenes, Republicans have painted targets on a mix of moderate-conservative Democrats and Democratic senators facing re-election next year in states Trump won just a few months ago.

Democratic leaders are keenly aware of the political reality for their colleagues.

"We want to wait and see a nominee. And if that nominee is not middle of the road, I'm afraid there's going to be resistance. I can't tell you every Democrat feels the same way, but enough of us do to make the point," the No. 2 Senate Democrat, Dick Durbin, told CNN.

Durbin said that some Democrats will break rank, but he is trying to head that off.

"That's always the question: Those who are vulnerable, in states that went strongly for Trump, you know, we've got to talk to them, make sure we understand their position," he said.

Democrats have not firmly said if they will filibuster a nominee -- and Republicans have not flatly said they would break that filibuster through a rules change known as the "nuclear option" -- but those cards are effectively on the table, weeks before Trump submits a nominee.

But the Trump team is still plotting for a possible climb that includes picking off at least eight Democrats, a tall order by any measure, much less a vacant seat on the Supreme Court.

Pence went to work on a group of six senators at the Capitol Wednesday and Trump aides have been working behind the scenes at the Capitol.

"Today was really about talking about our legislative agenda, but also meeting with members of the Senate to get their input on the president's decision about filling the vacancy on the Supreme Court," Pence told reporters last week.

Pence said he "hopes" moderate Democrats will come on board with his pick.

"The President-elect made very clear today we do expect -- he's not yet made a decision -- but we're in the process of winnowing that list now," Pence said.

The 60-vote bar has been somewhat informally set by Senate Minority Leader Chuck Schumer himself, who told MSNBC last week that he would "absolutely" do all he could to keep the Supreme Court seat open.

And the threat of a filibuster was clearly on the minds of lawmakers as Pence tested their feelings on the Supreme Court nomination.

With everything else swirling at the Capitol -- a wave of high-stakes confirmation battles and the opening of the Obamacare fight dominated the week -- the talks on the Supreme Court have taken on a surprising sense of urgency.

After he exited his meeting with Pence, West Virginia Democrat Joe Manchin refused to say if he would join a unified front of Democrats against any Trump nominee.

"I've pretty much been a West Virginia Democrat," Manchin said.

Montana Sen. Jon Tester -- another clear target of Republicans -- declined to flatly rule out supporting any Trump nominee, saying Wednesday that he would like to see who is being considered.

"I'll take a look at that list" of potential Supreme Court nominees, Tester said.

Trump said Wednesday he has a list of 20 names he is considering. And conservative judicial activists, led by the Federalist Society and the Heritage Foundation, have been advising Trump on who he should select.

Republicans are eight votes shy of a 60-vote threshold needed to break any filibuster on a Supreme Court nominee. But a handful of factors could swing the balance, starting with vulnerable Democrats seeking re-election in 2018.

At the top of that list are moderate-conservative Democrats like Manchin and Tester and others, including North Dakota Sen. Heidi Heitkamp and Indiana Sen. Joe Donnelly. Pence met with Manchin and Donnelly, and also Virginia Sen. Tim Kaine -- who could also face a tough re-election bid in a state that Hillary Clinton just narrowly won.

However, Senate Judiciary Committee Chairman Chuck Grassley said he was confident that Republicans wouldn't even have to wrestle with that and floated one of his party's key attacks against Democrats -- that they would look like hypocrites trying to block a nominee after complaining about the Republicans' treatment of Merrick Garland.

"You got to remember how much of a fuss they made after (former Justice Antonin) Scalia died that we weren't going to take it up because we were going to let the people take a vote. So between that and the election they can't complain about a longer period of time now," Grassley said.

The Supreme Court vacancy has been a particular sore point in the Senate, in particular, after Republicans refused to even consider President Barack Obama's selection to fill the vacancy left after former Scalia died last year. Some Democrats, including Oregon Sen. Jeff Merkley, have accused Republicans of "stealing" a Supreme Court seat by never considering Obama nominee Merrick Garland.

And Sen. Patrick Leahy, the former chairman of the Senate Judiciary Committee who presided over the last two appointments to the Supreme Court -- Sonia Sotomayor and Elena Kagan -- blasted Republicans when asked whether Democrats should filibuster.

"We're having a difficult time knowing what's going to happen because even though every Republican senator raise their hand swore unto God they would uphold the Constitution and then refused to for a year on advise and consent," Leahy said. "It's hard for me to predict what they might do. You see shown for the first time in the history of the country they are willing to violate the Constitution and politicize the Supreme Court. So what's going to happen now I don't know."

Senate Republican leaders are holding their cards tight when it comes to possibly breaking a filibuster by simply rewriting the rules to only require 51 votes to end the debate -- a move known as the "nuclear option."

But a top Senate Republican aide warned that Supreme Court fights have always been vicious and that Republicans have a long memory of all the times they have been stifled by Democrats.

"They made a term for it: Borked," said the Republican staffer, a reference to Robert Bork, who's nomination was squashed by Senate Democrats three decades ago -- ultimately paving the way for Justice Anthony Kennedy to join the court.

Even with the focus on Democrats in the battle, Senate Republicans still have some of their own hurdles -- highlighted by two of the senators Pence met with last week: moderate Sens. Susan Collins, of Maine and Lisa Murkowski, of Alaska.

Both women favor abortion rights -- which could easily become a sticking point amid promises from Trump that his appointees to the Supreme Court would overturn the landmark abortion ruling, Roe v. Wade.