Senate Democrats are privately acknowledging that President Trump will be very tough to beat in November if the economy stays strong and he draws on the substantial advantages of running as an incumbent.

Publicly, Democratic lawmakers are putting on a brave face, but behind closed doors anxiety is mounting over the unraveling of former Vice President Joe Biden's White House bid and the failure of impeachment to put a dent in Trump's approval ratings.

One of the chief concerns is that Trump, who has a virtually uncontested path to the Republican nomination, will have a big head start to prepare for the general election.

His reelection campaign is already spending heavily to reach out to voters on Facebook, and the Republican Party is solidly unified behind him.

Meanwhile, Sen. Bernie Sanders's (I-Vt.) victory in Tuesday's New Hampshire primary has made clear to some Senate Democrats that the party's primary is likely to drag on for months.

In particular, they worry the party will remain divided until the summer convention and fear a reprise of 2016, when lingering resentment among Sanders's supporters over the Democratic National Committee's favoritism toward eventual nominee Hillary Clinton dampened voter turnout in the fall.

"I hear comments all the time that after what's happened in the first two primaries we only have a 50-50 chance. It's not looking good," said a Democratic senator who requested anonymity to talk about the private concerns of colleagues.

One of the biggest surprises to lawmakers is the poor performance of Biden, who has performed well against Trump in hypothetical match-ups.

Though Biden led his Democratic rivals in national polls over the past several months, he finished in fourth place in the Iowa caucuses, with 15 percent of the vote, and dropped to fifth place in the New Hampshire primary, with a paltry 8 percent.

Other red flags are Trump's resilient approval rating, despite months of an impeachment inquiry followed by a weeks-long trial, and the amount of money his campaign is raising and spending.

The Democratic senator said mounting anxieties are "reflective of not just the candidacies but of how Trump is raising all this money and how he's really focused."

"They're targeting their people, they're going to get their people out," the lawmaker added.

An analysis by The Guardian newspaper found that Trump spent nearly $20 million on 218,000 Facebook ads in 2019, far surpassing the leading Democratic candidates.

That dynamic has only begun to shift recently as former New York City Mayor Michael Bloomberg has spent more than Trump on Facebook and Instagram ads since Jan. 1.

But Bloomberg, a billionaire who is self-financing his campaign, didn't compete in Iowa and New Hampshire, and it remains to be seen if the onetime Republican can energize Democratic voters.

Bloomberg has only recently emerged as a viable candidate, and some Democratic senators wonder how well a businessman who amassed tens of billions of dollars through a publishing company catering to Wall Street clients will play with the base.

Trump has also reported impressive fundraising numbers. The Republican National Committee and the Trump campaign said they raised $60.6 million in January and $525 million since January of last year.

The Democratic presidential field and the Democratic National Committee raised a combined $580 million in 2019, but much of what the candidates have raised has been quickly spent in the battle for the nomination.

Democratic candidates have also had to weather attacks from each other. Sanders and Sen. Elizabeth Warren (D-Mass.) are absorbing blows for their support of "Medicare for All," while former South Bend, Ind., Mayor Pete Buttigieg has been knocked for his lack of experience. Biden, meanwhile, has been forced to defend controversial votes he cast during a long Senate career.

Several Democratic candidates, including Biden, Sanders, Warren and Buttigieg, are beating Trump head to head in recent national polls, but the president had a small lead over every potential rival except Biden in Wisconsin, a key battleground state, according to a Marquette poll in December.

The chief worry among congressional Democrats is that if the party doesn't settle on a nominee until the convention in mid-July, Trump will have a substantial organizing advantage.

"You hear everybody talk, 'If we take all that time, how are they going to be able to get organized to combat what [the president] is doing?'" the senator said.

A second Democratic senator confirmed there is broad concern among colleagues over what they see as a difficult path to beating Trump and stressed that is why it will be extremely important for everyone to embrace the eventual nominee, even if that candidate is viewed by some as too liberal or too moderate.

"It is a concern because we had such a bitter divide four years ago," the senator said, referring to the misgivings Sanders supporters had over Clinton winning the nomination.

The lawmaker noted that the public has gotten used to what Democrats - and many Republicans - see as Trump's outrageous behavior while the president has racked up a string of recent accomplishments, such as the United States-Mexico-Canada Agreement, a truce in the trade war with China and strong employment numbers.

"It's given him a lot to talk about, and people have kind of become used to his misbehavior," the second senator said. "I think everyone's very nervous right now, and that nervousness is contributing to this sense of, whoever wins [the nomination], we've got to be there together."

Publicly, some Democratic lawmakers insist they're feeling optimistic after Iowa and New Hampshire, where Sanders, a candidate many of them view as less electable than Biden, tied for first and won outright, respectively.

"I love the fact that we had the highest voter turnout, I think, above 2008. That's very exciting," Sen. Debbie Stabenow (D-Mich.) said of the New Hampshire primary, where nearly 300,000 people cast ballots. "We're just beginning the primary process, but I think this is very positive."

Turnout in Iowa, however, was below 2008 levels.

Stabenow acknowledged the picture is muddled heading into Super Tuesday on March 3. Asked to name the front-runner at this point, she responded, "I don't think there is one."

But she argued that fellow Democrats need to calm down.

"As usual, we as Democrats are always panicked. And it's too early to panic," she said.

Democratic lawmakers, however, were dismayed by signs that Trump's approval ratings got stronger over the course of the impeachment process, which fired up the GOP base and failed to register as a priority among independents and swing voters - even though a large majority of them agreed with Democrats that new witnesses and subpoenas should have been considered at Trump's trial.

The president's approval rating has been trending up since the end of October, when Democrats had the most political momentum behind their impeachment inquiry after two associates of Trump's personal lawyer Rudy Giuliani were arrested at Dulles International Airport and former U.S. Ambassador to Ukraine Marie Yovanovitch testified before the House.

The president's approval rating dipped to 41.6 percent on Oct. 26 but has since climbed to 45.3 percent, according to an average of polls compiled by RealClearPolitics.

Trump registered a 49 percent job approval rating in a Gallup tracking poll in early February, the highest mark since he took office.

Sen. Ben Cardin (D-Md.) said he's surprised that Trump's conduct, which GOP senators such as Sen. Lamar Alexander (Tenn.) and Lisa Murkowski (Alaska) have called "inappropriate" and "shameful," hasn't had more of an effect on public opinion.

"I think it's not that things aren't going well for Democrats, I don't think that's the case. I think things are going better for Donald Trump than expected. I would have thought the people of this country would not want someone who lies as president, would not want somebody who demeans others," Cardin said.

"All these things are just against our values," he added. "I thought that in time would erode a significant amount of support, and although it has eroded some of his support [it's] much less than I thought."