Joe Biden on Sunday sought to derail a push by President Trump to swiftly install a new Supreme Court justice, making an unusually personal appeal to Senate Republicans to “cool the flames” and sidestepping calls from many in his own party for dramatic retaliatory measures such as expanding the size of the Supreme Court if Trump presses ahead.

Seeking to present himself as a president who would defuse the political and cultural battles engulfing the nomination process, Biden said that if he wins the November election he should be the one to fill the vacancy created by the death of Justice Ruth Bader Ginsburg, whom Trump and Senate Majority Leader Mitch McConnell (R-Ky.) have vowed to quickly replace.

Speaking in Philadelphia, Biden issued an extraordinary appeal to his former Senate colleagues, pleading with moderate Republicans to oppose Trump and McConnell and saying the vacancy presents those senators with a moral test.

“We need to de-escalate, not escalate,” Biden said. “So I appeal to those few Senate Republicans, the handful who really will decide what happens: Please, follow your conscience. Don’t vote to confirm anyone nominated under the circumstances President Trump and Senator McConnell have created. Don’t go there.”

Biden’s remarks contrasted sharply with the reaction of many Democratic activists who are enraged that McConnell is pushing an election-year appointment after blocking President Barack Obama’s nomination of Merrick Garland four years ago. Proposals like adding more justices to the Supreme Court, long considered fringe, are gaining more acceptance in the party’s mainstream, as many Democrats say Republicans are no longer playing by long-accepted rules.

Rather than weighing in on such ideas, Biden stressed less-polarizing issues — noting that a new conservative justice could threaten the Affordable Care Act, which currently faces a challenge before the Supreme Court, and strip away health-care protections during a pandemic. It was part of a synchronized message on the court and the ACA with top Democratic leaders on Capitol Hill. A Biden aide said both sides were in touch and would be coordinating regularly.

At a campaign rally in Fayetteville, N.C., Sept. 19, President Trump told supporters he planned to nominate a woman to the Supreme Court in the coming week. (The Washington Post)

While Trump has released a list of potential appointees to the court, Biden said Sunday he would not do so, suggesting it would inflame tensions and subject the prospects to political attacks. He reiterated his commitment to nominating an African American woman, and said that both Republican and Democratic senators would have input on his selection.

Supreme Court confirmation battles have been among the most explosive political events of recent years, culminating in the angry 2018 fight over now-Justice Brett M. Kavanaugh. The prospect of another such battle threatened to further roil the already bitter presidential campaign at a time when Biden is seeking to project the aura of a different kind of leader. With Sunday’s comments, he turned the Supreme Court vacancy into an early test of his philosophy that Democrats should reach out to receptive Republicans rather than match the GOP’s belligerence with their own — an approach that is rejected by many in the party’s liberal wing.

During his speech at the National Constitution Center, the former vice president stressed repeatedly that voting had already begun, saying that would make the rush to confirm a new justice an affront to democracy.

He offered a simple proposition: If Trump wins, the Senate ought to consider his pick. But if Biden wins, Trump needs to withdraw his nominee.

This is not the first time Biden has faced calls from the resurgent left of the Democratic Party to adopt a far-reaching position — from “Medicare-for-all” to “Defund the police.” Each time, he has quickly disavowed those positions, while promising to take strong action of his own.

On Sunday, he noted pointedly that Senate Republicans have not yet confirmed a nominee, and made it clear that he was focused on preventing them from doing so, rather than contemplating the consequences if they did. “I’m not going to assume failure at this point,” Biden said. “I believe the voices of the American people should be heard and will be heard.”

His comments were notable for urging Republicans to disregard the position of their party leaders, Trump and McConnell. “The last thing we need is to add a constitutional crisis that plunges us deeper into the abyss, deeper into the darkness,” he said.

Some on Biden’s team have been aggravated by the calls on the left to expand the court, expressing particular annoyance with Sen. Edward J. Markey (D-Mass.) for urging Democrats to abolish the filibuster and add more justices if Senate Republicans move to fill the vacancy.

“People in your own party shouldn’t cause you problems 44 days out,” said one adviser, who like others spoke on the condition of anonymity to be candid.

The notion of “court packing” has suffered from a bad reputation since President Franklin D. Roosevelt tried and failed in 1937 to expand the Supreme Court, frustrated that the current justices were rejecting many of his New Deal programs. But the Constitution does not specify a particular number of justices; there have been as few as five and as many as 10 over the nation’s history.

Outside pressure has been building for Biden to do and say more, especially about the kind of justice he would nominate. “He has to,” said Shaunna Thomas, the co-founder of UltraViolet, a women’s group. “He’s in a unique position as the candidate to help people understand exactly what is at stake.”

Biden advisers are determined not to bow to forces urging them to change their approach. They have long been content focusing voter attention on Trump’s widely criticized handling of the coronavirus pandemic, and are reluctant to get enmeshed in a fight that could alienate centrist voters. On Sunday, Biden aimed to draw a direct connection between the court battle and the virus, citing the threats a more conservative panel would pose to the ACA.

“If Donald Trump has his way, the complications from covid-19 . . . like lung scarring and heart damage, could become the next deniable preexisting condition,” Biden said.

Democrats fear a new conservative justice succeeding the liberal Ginsburg could tilt the court to the right and tip the outcome on high-profile cases for decades to come. Biden said the environment and the rights of voters, immigrants and women were also at stake. But he avoided specific mention of some divisive topics such as guns and only briefly made a reference to abortion rights.

Although Biden’s advisers believe that the vacancy could energize voters concerned about issues like health care and women’s rights, they do not plan to make the court a primary part of their campaign message.

“It highlights the importance of the role of the president,” said Rep. Cedric L. Richmond (D-La.), co-chairman of Biden’s campaign. “This will double down on the seriousness of this election. If the 200,000 covid deaths didn’t do it, this will do it.”

This tempered view of how to handle the vacancy is partly due to polling reviewed by the Biden team showing that most voters are not familiar with the Supreme Court’s activities. “You can expect it to come up in a way that reinforces the core message, but we’re not going to reorient our campaign around a set of talking points that only resonates with D.C. insiders and court watchers,” said one Biden adviser.

Republican leaders, by contrast, hope a divisive battle will revive their flagging efforts to keep the presidency and the Senate in the November election by providing GOP voters who have soured on Trump incentive to put aside their differences.

“Biden knows that he is an empty vessel for the radical left, and that’s why he’s refusing to be honest with the American people about who he would want on the court,” Trump campaign spokesman Tim Murtaugh said in a statement.

While Biden has said he has been working on a list of potential nominees, he defended his decision not to publicize it. “Putting a judge’s name on a list like that could influence that person’s decision-making as a judge — and that would be wrong,” he said.

After committing during the primary to appointing a Black woman, he has rarely talked about the Supreme Court.

The Democratic presidential nominee steered clear of another Republican target Sunday by making no mention of expanding the court. While he has not referenced it explicitly since Ginsburg’s death, Biden has in the past been adamantly opposed to the idea.

“I think it’s a bad idea,” he said in August 2019 during a trip to Iowa. “It will come back to bite us. It should not be a political football.”

Whether Biden and the Democrats can stop Republicans from confirming Trump’s forthcoming nominee remains an open question. They received a boost on Sunday when Sen. Lisa Murkowski (R-Alaska) joined Sen. Susan Collins (R-Maine) in opposition to taking up a nominee so close to the election. Under the current balance of the Senate, Republicans can afford only one more defection, assuming the entire Democratic caucus stays unified in opposition.

One area where the Biden team believes the vacancy could be helpful is among women under 40, who have been slow to rally behind to his candidacy. The galvanizing issue for many of these voters is the possibility of losing abortion rights if Roe v. Wade, the landmark 1973 case guaranteeing the right to an abortion, is overturned.

Outside the campaign, some Democratic-aligned groups plan to focus more intently on the court fight over the next few weeks. Leaders of women's groups and liberal organizations immediately began discussing a 50-state march intended to echo both the 2016 Women’s March and the demonstrations in 2018 after Kavanaugh joined the court.

“A real uprising, that has got to be [Ginsburg’s] legacy,” said Hilary Rosen, a Democratic strategist close to the Biden campaign and the co-founder of Times Up Legal Defense Fund, an organization to help survivors of sexual harassment.

The demonstrations will be geared toward motivating voter turnout and might be organized as marches to the polls, Rosen said.

Ginsburg’s death, and the resulting vacancy, has already had an enormous impact on the party’s fundraising. ActBlue, a clearinghouse for Democratic donations, announced Sunday it had raised $100 million from small-dollar donors since news broke Friday night of Ginsburg’s death.

Felicia Sonmez contributed to this report.