• U.S. President Joe Biden's decision to drop out of the presidential race on Sunday and to endorse Vice President Kamala Harris as the Democratic nominee, gives Harris very little time to turn around her mixed political standing.
• With just over 100 days until the presidential election in November, Harris now has the hard task of winning the Democrats' official nomination and of winning over her detractors and undecided voters.

The pressure is on Vice President Kamala Harris to turn around her political fortunes and win the Democratic Party's nomination, analysts say, after U.S. President Joe Biden dropped out of the presidential race.

With just over 100 days until the presidential election in November, Harris now has to win over not only Democratic delegates, donors and secure an official nomination by the party, but also — and most crucially — her detractors and undecided voters.

"This is not a smooth road ahead for Democrats. Donald Trump and [running mate, Sen.] JD Vance are a very strong ticket that clearly appeal to a large segment of America, but I think Kamala Harris can give them a good run for their money," Lew Lukens, senior partner at Signum Global Advisors, told CNBC on Monday.

"I'm not saying it's going to be easy for Kamala Harris ... but I do think we have to give her the opportunity to be the candidate," he noted.

"The vice presidential role, by its very definition, is very much behind the scenes, playing second fiddle to the president. Now she's suddenly been thrust into the spotlight [as the person] who will most likely be the [Democratic] nominee, so it's a very new role for her and we'll have to see if she grows into that role," Lukens told CNBC's "Squawk Box Europe."

Harris is the first woman, and the first Black and Asian American person, to serve as vice president in the U.S., with her background boosting her appeal among some sections of the electorate.

Still, the similarity between Harris' and Biden's poll ratings suggests that, even if she wins the Democratic presidential nomination, she will need to convince voters she offers something different to Biden.

"As a woman, an African-American and Asian woman, she has attributes that will work in various kinds of constituencies, plus the fact she's much younger and they're hoping that will have an effect. But there are very, very much deeper questions which had alienated a lot of Democratic voters … [and] a lot of people were not committed," Inderjeet Parmar, professor of international politics at City, University of London, told CNBC on Monday.

Parmar added that, if nominated, Harris will have to convince voters that she has something "significantly different" to offer than Biden put forward in his campaign.

Harris' performance as vice president is widely judged to have been underwhelming, although analysts acknowledge the 59-year-old former senator from California was given something of a poisoned chalice early in her tenure when she was handed the difficult brief of tackling illegal immigration at the U.S.-Mexico border.

Her prominence in the public sphere appeared to decline early in Biden's presidency as she became a frequent target for Republicans and the right-wing media. Despite her stance on racial equality, gun crime and women's reproductive rights winning her supporters among younger voters, women and minority groups, Harris' popularity ratings have remained stubbornly low.

Recent voter polls, conducted before Biden announced he would not run for reelection, broadly showed Harris on par or slightly ahead of Biden, but lagging behind Republican nominee Donald Trump.

The latest NBC News poll, conducted before a failed assassination attempt on Trump on July 13, showed both Biden and Harris trailing Trump by 2-point margins among registered voters, with similar results seen in other recent voter surveys.

Attention in the U.S. is now concentrated on how much political and financial support Harris can muster ahead of the Democratic National Convention in Chicago in August. The vice president's newly launched campaign raised $49.6 million in grassroots donations in under one day, according to comments by a campaign spokesperson for Biden carried by Reuters on Monday.

Harris now has to formally secure the nomination from around 4,000 Democratic convention delegates who had previously backed Biden. Although she has been endorsed by Biden, and is the front-runner to receive the party's nomination, it is not guaranteed as challengers could emerge.

Over the last 24 hours, high-profile Democrats, including former President Bill Clinton and former Secretary of State Hillary Clinton, have voiced their support for Harris' nomination

Nonetheless, it hasn't escaped notice that former President Barack Obama and former House Speaker Nancy Pelosi did not immediately throw their weight behind the vice president, suggesting that some high-profile Democrats might need convincing that Harris is the right candidate to beat Trump.

Signum Global Advisors' Lukens said Harris' approval ratings within the Democratic Party remained high at around 85%, but her main challenge is to transmit this to a broader range of the electorate.

"Democrats like Kamala Harris, the question is whether she can translate that popularity with Democrats to a broader popularity that will attract independent and swing voters across the country," he noted.

No choice but to unite?

Political analysts have said it would be a big risk for the Democratic Party to consider other presidential nominees at this late stage in the election process.

"The Democrats would be committing virtual suicide by having a big party fight and finding someone more quote unquote more electable than Kamala Harris," Allan Lichtman, presidential historian and distinguished professor of history at the American University, told CNBC on Monday.

"If Democrats grow a spine, and become smart, they will unite behind Harris," said Lichtman, who is also one of the creators of the "Keys to the White House" prediction model for determining the outcome of presidential elections.

"It would be [a] disaster for the Democrats to have an open party fight and party brawl. That would recreate exactly the conditions of 2016 that led to the election of Trump in the first place," he said.

"If Democrats have any hope of winning at all, unite behind Harris and don't think you can find some other, more quote-on-quote electable candidate," he told CNBC's "Capital Connection."

The "Biden for President" campaign committee has officially been renamed the "Harris for President" committee, affording Harris the use of Biden team's infrastructure, staff and its $98 million in cash reserves.

Political analysts say it's too soon to tell the extent of party unity around Harris' nomination and warn that divisions could cost the Democrats the election.

"If the Democrats can be unified and come out of this convention speaking with one voice, energized and excited, then they have a good chance to win in November," Steven Okun, senior advisor at McLarty Associates, told CNBC on Monday.

"If there is disunity coming out of this convention, if there are fights between the moderates and the liberals and the progressives and there [are] bad feelings and people aren't energized and stay home, then the Democrats are going to lose."