The United States will on Tuesday witness one of the tightest presidential elections in decades with Republican nominee Donald Trump and Democratic leader Kamala Harris making last-ditch efforts to sway the remaining undecided voters in key battleground states.

As the countdown for the election day began, Trump raked up the bitter memories of the 2020 election and said he "shouldn't have left" the White House, triggering apprehensions that he may not accept the outcome of the November 5 voting if he loses the race to Harris.

More than 75 million Americans have already cast their votes as of Sunday, according to the University of Florida's Election Lab that tracks early and mail-in voting across the US.

In the overall campaign, Harris has been projecting the election as the one to protect the country's fundamental freedoms, safeguard constitutional values and ensure women's rights, while Trump has been promising to rebuild the economy and rid the US from illegal immigrants.

Vice President Harris and former President Donald Trump are criss-crossing politically pivotal battleground states, making closing arguments to drum up support as various polls continued to predict an exceptionally tight race between two contenders.

However, a new poll in Iowa said Harris is leading with 47 percent as against Trump's 44 percent. Trump has swiftly rejected the poll.

Political analysts say either of the two leaders wrestling at least four of the seven battleground states may take the reins at the White House in January.

Questioning the counting process in the 2020 presidential election, Trump said at a rally in Pennsylvania on Sunday that he shouldn't have "left" the White House.

Following the election that brought Joe Biden to power, Trump alleged fraud in the voting process and challenged the outcome in courts which rejected his claims.

In his address, Trump also questioned the immigration policy of the Biden administration and said the country's borders were safe till he was at the White House.

"We had the safest border in the history of our country the day that I left. I shouldn't have left. I mean, honestly, because ..we did so well," the former President said at a rally in Lititz, Pennsylvania.

Trump refused to concede the 2020 election and unsuccessfully filed a series of court cases in challenging the outcome.

In the rally, Trump also targeted Harris and accused the Democratic Party of being a "corrupt machine".

"It's all corrupt. She is corrupt. She is a corrupt person. I am running against a totally corrupt person," he alleged.

"I am really not running against her. I am running against a corrupt machine called the Democrat Party." Pennsylvania has emerged as the most crucial of the seven battleground states having 19 electoral college votes followed by 16 each in North Carolina and Georgia, 15 in Michigan and 11 in Arizona. Other battleground states, Wisconsin has 10 and Nevada has six.

In the rally in Pennsylvania, Trump promised to usher in "a new golden age" in the US if he becomes the president and that he will "fix" the misdeeds of the Biden-Harris administration.

Trump is also holding rallies in North Carolina and Georgia while Harris is touring Michigan.

In her address at an event in Detroit, Harris said it is incumbent on the American people to decide the future course the United States takes, suggesting that Trump would be detrimental to the country.

Election Day offers voters the chance to reject "chaos, fear and hate", she said.

"In two days, we have the power to decide the fate of our nation for generations to come," she said.

"I see a nation determined to turn the page on hate and division and chart a new way forward. As I travel, I see Americans from so-called red states and so-called blue states who are ready to bend the arc of history toward justice," Harris added.

The Democratic Party leader also invoked God and said His plan is to "heal us and bring us together as one nation" but that it is not enough, adding "we must act" to realise the plan.

Except for the headline, this story has not been edited by The Telegraph Online staff and has been published from a syndicated feed.