Both Senate Majority Leader Mitch McConnell and President Donald Trump want a vote on his nominee to replace Supreme Court Justice Ruth Bader Ginsburg -- but they're not expected to find that easy going.

The GOP effort could be thwarted by as few as four Republicans siding with Democrats.

One of them, Sen. Susan Collins of Maine, said Saturday that any confirmation vote should wait until after the presidential election and that the man elected Nov. 3 should select the nominee.

"I do not believe that the Senate should vote on the nominee prior to the election," Collins, who is in a fierce re-election battle, said in a statement Saturday. "In fairness to the American people, who will either be re-electing the President or selecting a new one, the decision on a lifetime appointment to the Supreme Court should be made by the President who is elected on November 3rd."

Sen. Lisa Murkowski, R-Alaska, is expected to join Collins in opposition. Both women support abortion rights, and both have stated that they believe how the GOP-controlled Senate handled President Barack Obama's nominee, Judge Merrick Garland, in 2016 sets a precedent that ought to be followed in 2020.

"When Republicans held off Merrick Garland it was because nine months prior to the election was too close, we needed to let people decide. And I agreed to do that," Murkowski told The Hill this summer. "If we now say that months prior to the election is OK when nine months was not, that is a double standard and I don’t believe we should do it."

Other Republicans who will be watched closely in the coming weeks have not yet made their positions known, seemingly, unlike Collins, following the advice of McConnell who Friday advised his members to “keep the powder dry” in a letter obtained by the Washington Post and confirmed by ABC News.

Eyes will be on Sen. Mitt Romney, who was the lone Republican to vote in favor of Trump’s impeachment at the beginning of the year.

Senate institutionalists, like Lamar Alexander of Tennessee, as well as those up for reelection and especially vulnerable, like Cory Gardner, R-Colo., could also voice opposition.

A GOP source with knowledge of the matter tells ABC News that conservatives are pushing President Trump to name a replacement soon in order to "put a face and a personal story" behind their push to replace Justice Ginsburg. The source said that could make it harder for some to oppose the nominee.

Trump said Saturday afternoon he "totally disagrees" with Collins.

"Well, I totally disagree with her. We have an obligation. We won. And we have an obligation as the winners to pick who we want. That's not the next president. Hopefully, I will be the next president," he told reporters as he left the White House for a campaign rally in North Carolina.

When a reporter asked, "What about Obama?" Trump responded, "But we're here now, right now we're here, and we have an obligation to the voters, all of the people, the millions of people that put us here, in the form of a victory. We have an obligation to them, to all of those voters, and it's a very simple thing. So, I would disagree. If that's what she said, that's not the way I read it. I read it differently, but, but if that's what she said, I totally disagree."

Once Trump has named his pick, which he said would come "next week," it will be up to Senate Judiciary Chairman Lindsey Graham to shepherd the nominee through his committee.

Graham announced Saturday that he will take up a Trump nomination just 45 days to the ​presidential election, reversing his past position on the matter, doing so in an election year when control of the Senate hangs in the balance.

“I will support President @realDonaldTrump in any effort to move forward regarding the recent vacancy created by the passing of Justice Ginsburg," Graham, of South Carolina, tweeted Saturday.

That about-face also puts Graham in line with McConnell, who announced in a statement Friday evening that Trump’s nominee “will get a vote on the floor of the U.S. Senate” shortly after the news broke of Ginsburg’s death.

Both men famously blocked Obama's nomination of Garland, his pick to fill the vacancy left by the death of Justice Antonin Scalia almost 11 months before the 2016 presidential election.

In 2016 Graham argued that he believed the vacancy came too close to a presidential election at a time when the Senate and the White House were controlled by different parties.

“If there is a Republican President in 2016 and a vacancy occurs in the last year of the first term, you can say ‘Lindsey Graham said let us let the next President, whoever it might be, make that nomination and you could use my words against me, and you would be absolutely right,” Graham said at the time.

McConnell also said in 2016 that Garland’s nomination came too close to a presidential election. Later, the Kentucky senator went on to say that he opposed pushing a nomination through because the Senate was controlled by Republicans at the time, while the White House was held by a Democrat.

In a speech in his home state, McConnell said that telling Obama that his nomination would not clear the Senate was “one of the proudest moments” of his Senate career.

Senate Democrats and their outside allied groups have blasted the shift by McConnell and are expected to fight to keep a third Trump nominee from the highest court in the land.

Minority Leader Chuck Schumer told Senate Democrats in a conference call Saturday afternoon that their "number one goal" must be to communicate what is at stake in the upcoming battle to fill the seat, according to a source familiar with the call.

Still, because of a rules change engineered by Republicans in 2017, Democrats have no real procedural tools at their disposal to stop the GOP majority from pushing through a nomination.

In 2017, Republicans altered the rules of the Senate to allow a simple majority of the chamber to approve a Supreme Court nomination -- at the time, conservative jurist Neil Gorsuch -- down from the 60-vote threshold previously needed for high court appointees and legislative measures to move forward to a final vote.

Conservatives are pushing for the eventual nominee to be confirmed by Election Day, something several GOP leadership aides and former officials responsible for the Senate nomination process said would be nearly impossible.

On average, a nomination takes 70 days from nomination to final Senate confirmation, according to a 2018 Congressional Research Service Report. That said, John Paul Stevens waited just 19 days, followed by Sandra Day O’Connor whose nomination lasted just over a month, the report noted.