Kamala Harris is set to become the Democratic presidential nominee and is likely to pick a “white guy” running mate to boost her chances in key battleground states.

The vice president, 59, raised a record-breaking $80 million in the 24 hours after Joe Biden stood down, as her potential rivals ruled themselves out of the contest and endorsed her.

Nancy Pelosi, a major powerbroker in the party, broke her silence to back Ms Harris on Monday, after working behind the scenes to end Mr Biden’s run.

The former House speaker said she had “limitless optimism for our country’s future” under a Harris administration and described her as a “champion for working families” and abortion rights.

Sources said on Monday night that Ms Harris’s campaign believed she could secure enough votes for her nomination by Wednesday.

Ms Harris is now thought to be considering a male candidate from the midwestern US as her vice-presidential candidate, amid concerns her background as a liberal Californian politician could discourage blue collar voters from supporting her.

Steve Schmidt, a Democrat-affiliated strategist, told Puck News on Monday: “It can’t be two female candidates. It can be, but I don’t think that’s going to go. So it’s going to be a white male candidate.”

Polling shows that Ms Harris is behind Donald Trump in the key midwestern “blue wall” states of Michigan, Pennsylvania and Wisconsin, and in all other swing states across the country.

Andy Beshear and Josh Shapiro, the governors of Kentucky and Pennsylvania, are thought to be under consideration to be her running mate. Pete Buttegieg, a former presidential candidate from Indiana now serving as Mr Biden’s transportation secretary, is a third option.

One of the few women thought to be a possible vice president candidate, Gretchen Whitmer, ruled herself out of the contest to be a running mate after she was appointed co-chair of Ms Harris’s last-minute presidential campaign.

JD Vance, who was named Donald Trump’s running mate last week on the expectation he would perform well with blue collar midwestern voters, was set to deliver his first solo remarks on the campaign trail in Middletown, Ohio on Monday night.

The battle for working class voters, who contributed significantly to Trump’s win in 2016, is set to become one of the main themes of the 2024 race, after Mr Biden’s attempt to win back support over the last few months.

Ms Harris is considered more Left-wing than Mr Biden on some policy areas, including on immigration and healthcare. During her unsuccessful 2020 presidential campaign, she called for a “fundamental overhaul” of the immigration system to make it less “cruel”.

Although the process for selecting a new nominee is technically open after Mr Biden dropped out, her path appears to be clear after she received the endorsement of more than 1,000 delegates to the Democratic National Convention, more than 180 congressmen and several key trade unions.

Future Forward, a political action committee (PAC) that previously supported Mr Biden, said on Monday that it had raised $150 million in support for Ms Harris in the previous 24 hours.

A separate $80 million was raised from small-dollar donors through the ActBlue fundraising platform, the organisation said. The speed of donations to Ms Harris is thought to be a record.

She will also inherit around $91 million in leftover campaign funds donated to her joint ticket with Mr Biden.

Senior Democrats are considering running a “virtual roll call” that would install her as the party’s nominee before the convention in late August.

However, Mike Bloomberg, a major Democratic donor, urged the party to take its time in selecting a nominee to replace Mr Biden.

“We don’t need to get a resolution right away, but we do need to get it right,” he wrote in a statement on Twitter. “The decision is too important to rush, because the election is too important to lose.”

In her first speech since Mr Biden dropped out, Ms Harris praised his “unmatched legacy” in office and said he had “already surpassed” the record of many two-term presidents.

Speaking about her friendship with the president’s deceased son, she said: “The qualities that Beau revered in his father are the same qualities that I have seen every day in our president: his honesty, his integrity, his commitment to his faith and his family, his big heart and his love, deep love of our country.

“I am first-hand witness that every day our president, Joe Biden, fights for the American people, and we are deeply, deeply grateful for his service to our nation.”

Mr Biden, who is self-isolating with Covid-19 at his home in Delaware, has not appeared publicly since his letter announcing the decision to stand down on Sunday afternoon.

He has rejected calls from some Republicans to resign the presidency immediately, amid concerns about his health and age.

Ms Harris is unlikely to make a decision about her running mate until after she receives the nomination. Representatives for Mr Beshear and Mr Shapiro did not return requests for comment about their interest in becoming a vice presidential candidate.

Ms Harris’s campaign, which is largely staffed by people who worked for Mr Biden, released its first attacks on Trump on Monday.

The campaign looks set to focus on Trump’s conviction in the “hush money” case in late May, while emphasising her previous role as a district attorney in California.

“Kamala Harris stands up to fraudsters and criminals,” the campaign said, adding: “Donald Trump is a convicted felon.”

Ms Harris, who is more than 20 years younger than Mr Biden, is also planning to launch a series of attacks on the 78-year-old Republican’s age.

Erin Wilson, her deputy chief of staff in the White House, told congressmen: “Her role as a prosecutor makes her the ultimate contrast to Trump, the convicted felon. She can make the issue of age and fitness a liability for Trump.”

Ms Harris herself spent Sunday afternoon speaking to more than 100 Democrats on Capitol Hill as she shored up support in the party after Mr Biden’s decision.

Trump’s campaign, which has complained about the switcheroo between Mr Biden and Ms Harris, released a statement describing her as “Biden 2.0”.

The campaign accused her of “defunding the police” while working as a district attorney and senator in California.

“Harris supported Los Angeles’s cuts to their police department, called efforts to add more police to the streets ‘wrongheaded thinking,’ and opposed the death penalty for cop killers,” the campaign said in an email on Monday.

In a post on Truth Social, Trump called the Democrats a “threat to democracy”.

Legal experts said that a Republican plan to launch legal challenges to Ms Harris’s nomination in some states looked unlikely to succeed.

Lawrence Otter, an electoral lawyer, told The Telegraph: “I wouldn’t be surprised if they tried it. But is it going to go anywhere? Highly unlikely.

“Most state courts have some case law that will tell them that they have to have a liberal interpretation of their election code to allow the voters to have a choice.”