Senate Democrats are divided on how aggressive they should be in opposing President Trump's Supreme Court nominee, with at least two red-state centrists up for re-election trying to put the brakes on their filibuster-bent caucus.

Sen. Joe Manchin, D-W.Va., on Tuesday signaled that he would break with his party's leadership if they decided to mount a filibuster, a move Sen. Jeff Merkely, D-Ore., has all but promised against anyone Trump would nominate to the high court.

"I am not going to filibuster anybody," Manchin told reporters Tuesday before Trump announced who would be his choice to replace the late Antonin Scalia on the high court.

North Dakota Sen. Heidi Heitkamp, a Democrat whose state voted overwhelmingly for Trump, also appeared to be urging a far more measured approach.

Heitkamp told Politico that Trump's nominee "should get a full hearing," and should "absolutely" receive an up-or-down vote on the Senate floor rather than face a Democrat-led filibuster blockade.

But Democrats who serve on the Judiciary Committee appeared to be hardening their stance toward Trump's high court pick – even before the name was announced – in response to anger over Trump's executive action requiring a temporary halt on immigration and travel from seven majority-Muslim countries.

Sen. Richard Blumenthal, D-Conn., said Trump's decision to fire acting deputy attorney general Sally Yates Monday after she said she would not enforce the immigration action, "reaffirms very powerfully and dramatically the need for someone who will be in the mainstream and who will have an unquestionable allegiance to the rule of law and the U.S. Constitution."

"So I will be scrutinizing the nominee even more closely because it is one of the most important decisions that we are going to be making here," he told the Washington Examiner.

If Democrats mount a filibuster, Republicans will face pressure from Trump to go nuclear when it comes to Supreme Court nominees and change Senate rules to approve high court picks with a simple 51-vote majority, instead of the 60 votes now needed to overcome a potential blockade.

Senate Democrats went "nuclear" in 2013 with respect to most presidential nominations, changing the chamber's rules to win confirmation with just a 51-vote majority. They excluded Supreme Court nominations from that lower vote threshold, a decision they are now urging Republicans to sustain.

To end a likely filibuster and avoid having to decide whether to "go nuclear" or not, Republicans would need to persuade eight Democrats to buck their party and join them in moving forward with a high court pick's nomination.

Invoking the nuclear option on high court nominees could come back to haunt Republicans down the road when Democrats control the White House again and have a chance to fill Supreme Court seats. Senate Majority Leader Mitch McConnell, R-Ky., along with other Republicans, is keenly aware of the turnabout and has voiced resistance to invoking the nuclear option for Trump's nominee.

Democratic leaders Tuesday also suggested that whoever Trump selects ought to meet the 60-vote threshold that President Obama's nominees were able to achieve by attracting bipartisan support. The Senate confirmed Justice Sonia Sotomayor in 2009 by a 68-31 vote and Justice Elena Kagan in 2010, 63-37.

Senate Minority Leader Chuck Schumer on Tuesday defended the 60-vote requirement to advance a high court nominee and said getting to 60 is a way for Trump to demonstrate that his nominee is "mainstream."

The New York Democrat said Senate Democrats don't regret changing Senate rules in 2013 to allow for all presidential nominations to pass on a simple majority vote except the high court, which still requires 60 votes to overcome a filibuster.

"We explicitly put forward that 60 should stay on the Supreme Court, and the reason for that is that this is such an important position, it ought to be bipartisan and mainstream as far as the nominees," he said.

"President Trump's nominees ought to meet the same threshold as President Obama's nominees did, which is the 60-vote threshold," Blumenthal added.

Minority Whip Dick Durbin, D-Ill., another member of the Judiciary panel, said Republicans' decision to hold up Obama's choice of Merrick Garland to fill the Supreme Court vacancy left by the death of Antonin Scalia is "still a painful" memory, but Democrats will at least allow whomever Trump chooses to have a hearing.