• Montana Democratic Sen. Jon Tester urged President Joe Biden to drop out of the 2024 election contest against former President Donald Trump.
• Biden faces mounting pressure by leading Democrats to step aside for a new nominee.
• The pressure on Biden stems from concerns that he could not only lose the White House, but also cost the Democratic Party party its Senate majority and doom any chance it has of regaining a House majority.
• Former President Barack Obama has privately expressed concerns to Democrats about the viability of Biden's candidacy,

Montana Democratic Sen. Jon Tester on Thursday night urged President Joe Biden to drop out of the 2024 election contest against former President Donald Trump, adding to already intense pressure from Biden's fellow Democrats to step aside for a new presidential nominee.

Tester, who faces a tough re-election fight in a state that Trump won in the past two presidential elections, is the second U.S. senator to call on Biden to quit the race.

Sen. Peter Welch, D-Vt., became the first senator do so last week.

Rep. Adam Schiff of California, the Democratic nominee for a Senate seat from that state, on Wednesday asked Biden to step aside as the nominee, joining 18 other Democrats in the House of Representatives who previously did so.

Later Thursday, Rep. Jim Costa, D-Calif., became the latest House member in that group, saying, "For the good of the country, I think it is time for the President to pass the torch to the next generation to carry on the legacy he started."

"Democrats need to unite and deliver their strongest team to the American people in this election," Costa said.

Former President Barack Obama has privately expressed concerns to Democrats about the viability of Biden's candidacy, both the Associated Press and The Washington Post reported.

Biden served two terms as Obama's vice president, and the 44th president still has unrivaled influence within the Democratic party.

The two Democratic leaders in Congress — Senate Majority Leader Chuck Schumer and House Minority Leader Rep. Hakeem Jeffries, both of New York — have told Biden in recent days that his presence on the party ticket could cost them majorities in both chambers of Congress.

Tester, in a statement Thursday, said, "Montanans have put their trust in me to do what is right and it is a responsibility I take seriously."

"I have worked with President Biden when it has made Montana stronger, and I've never been afraid to stand up to him when he is wrong," Tester said. "And while I appreciate his commitment to public service and our country, I believe President Biden should not seek re-election to another term."

Tester is seeking a fourth term in a contest against retired Navy SEAL Tim Sheehy, the Republican nominee. The race is one of several considered crucial for Democratic Party efforts to retain its majority control of the Senate.

Schumer was notified ahead of time that Tester would call on Biden to quit the election contest, a source with knowledge of the situation told NBC News.

Schumer then told Tester to do whatever he thought was best, according to that source.

Biden, who is isolating at his beach house in Rehoboth, Delaware, after testing positive for Covid-19 during a campaign swing through Las Vegas on Wednesday, for weeks had flatly rejected calls to step aside is now said to be more open to listening to top Democrats about the risk of him remaining in the race. He has also reportedly asked advisers in recent days whether they believe his vice president, Kamala Harris, could beat Trump in November.

"We're close to the end," a person close to Biden told NBC News.

The pressure on the 81-year-old Biden stems from concerns that after his June 27 debate, if he remains the nominee he will not only cost Democrats the White House, but also cost the party its majority in the Senate and doom its chances of retaking the House.

Sen. John Hickenlooper, D-Colo., told the Reuters news service late Wednesday that Biden is "working towards" a decision that will "put the country first." Hickenlooper did not explicitly call on Biden to drop out, saying that was "his decision to make."

"But certainly there's more and more indications that that would be in the best interests of the country, I think," the senator said.

Schiff, who is considered a strong favorite to win his race, is close to his fellow California Democrat, Rep. Nancy Pelosi, the former House speaker.

CNN reported Thursday morning that Pelosi told Biden recently that he cannot beat Trump and that he could doom Democratic chances of winning a House majority if he insists on remaining in the race.

Sen. Bob Casey, a Pennsylvania Democrat who to date has fully supported Biden's plan to stay in the race, told NBC News on Thursday he had heard "growing concerns" from voters in his state this week.

"I've talked to a lot of people in our state that have concerns ever since the debate," Casey said. "But I think my position has been very clear, and I think I think the president will do what he's always done, which is put the best interests of the country first."

The Biden campaign's public response to growing concerns has not changed, and top staffers remain dead-set against the president dropping out.

"Our campaign is not working through any scenarios where President Biden is not the top of the ticket," Quentin Fulks, principal deputy campaign manager told reporters in Milwaukee on Thursday.

"He is and will be the Democratic nominee," said Fulks.

Later Thursday, a source close to Biden pushed back at the top Democrats pressuring for the president to bow out.

"Can we all just remember for a minute that these same people who are trying to push Joe Biden out are the same people who literally gave us all Donald Trump?" the source told NBC News.

"In 2015, Obama, Pelosi, Schumer pushed Biden aside in favor of Hillary; they were wrong then and they are wrong now," the source said.

That source also noted how polls in 2016 showed the Democratic Party's presidential nominee Hillary Clinton leading Trump by as much as 9 percentage points.

"How did all this work out for everyone in 2016?" the source said, referring to Trump's victory that year.

"Perhaps we should learn a few lessons from 2016; one of them is polls are BS, just ask Sec. Clinton. And two, maybe, just maybe, Joe Biden is more in touch with actual Americans than Obama-Pelosi-Schumer?" the source added.