Hillary Clinton has begun a tour of key states in her campaign to become US president following her announcement that she intended to make a second attempt at becoming the country's first female commander-in-chief.

The former US senator, secretary of state and first lady tweeted that she was leaving for Iowa, the first US state to vote when the primary elections to choose each party's candidate begin early next year.

Clinton's first campaign event in Iowa is on Tuesday. Her van stopped en route in Pennsylvania where she met a family from Michigan, aide Huma Abedin said in a conference call, adding the road trip had been Clinton's idea.

In her announcement on Sunday that she would seek the Democratic nomination, Clinton promised to be a champion of "everyday Americans" if she made it to the White House.

"Everyday Americans need a champion, and I'm going to be that champion," she said in a video posted on her website, as she warned "the deck is still stacked in favour of those at the top" despite an economic recovery.

Speaking to camera at the end of a three-minute clip featuring personal stories from Americans of different ages, ethnicities and sexualities, Clinton pledged to ensure people could "get ahead and stay ahead" rather than "just get by".

It marked the official start of Clinton's final attempt to crack what she called the "the hardest, highest glass ceiling" after being defeated by Barack Obama in their bitterly fought contest for the Democratic nomination in 2008 and brought an end to years of speculation over whether Clinton would make a second bid to secure a place in history as the first female US president.

Clinton, 67, is seeking a fourth chapter in what is already an unprecedented career in American politics. She previously served as secretary of state and US senator for New York, as well as first lady when her husband, Bill, served two terms at the White House in the 1990s.

It is a record that makes Clinton one of the most enduring – and polarising – figures in American politics.

Clinton's campaign aides concede that that her familiarity to voters and decades of experience on the frontline of politics are both an asset in her bid for the White House and a potential weakness, allowing her to be portrayed as a candidate from the past.

Clinton returns to smash glass ceiling, with gender at forefront of campaign

Jeb Bush, the former governor of Florida, who is brother and son to recent White House occupants George W and George HW Bush, is currently among the frontrunners in the crowded field of aspiring Republican candidates. His nomination could turn the 2016 presidential election into a battle between two of the most familiar – and controversial – family names in modern US history.

Moving immediately to counter suggestions that she expects a so-called coronation from her party this time, Clinton stressed in her video that she was returning to the campaign trail to "earn your vote".

Clinton immediately began appealing for campaign volunteers in the important primary states of Iowa, Nevada, New Hampshire and South Carolina, as well as in New York, where her campaign is to be headquartered in the Brooklyn Heights neighbourhood.

A campaign strategy document written by Clinton's 35-year-old campaign manager, Robby Mook, and leaked to the press on the eve of her announcement instructed her staff to "take nothing for granted".

"We are humble," the memo said. "We are never afraid to lose, we always out-compete and fight for every vote we can win. We know this campaign will be won on the ground, in states."

Clinton's trip to Iowa on Monday, her first as an official candidate, will see her return to the state where she was pushed into third place by Obama and Senator John Edwards when it held its first-in-the-nation caucus seven years ago, a humiliating setback from which she was unable to recover.

Democrats in Iowa have reservations about Clinton, and are hostile to the idea that their role is simply to anoint the candidate-in-waiting whom they rejected previously. But the lack of any credible Democratic challengers to Clinton, who will draw on a supremely well-organised and well-funded campaign structure, make an upset extremely unlikely.