WASHINGTON -- Hillary Rodham Clinton promptly set off on the campaign trail after making her long awaited entry into the 2016 presidential race, and her potential Republican rivals wasted no time offering blistering criticism of the former secretary of state.

Clinton, seeking to become America's first female president, announced her candidacy Sunday and left on a roughly 1,600-kilometre trip from her New York home to Iowa, the Midwestern state that kicks off the long, state-by-state contest for the Democratic nomination.

In a video message announcing her candidacy, Clinton promised to serve as the "champion" of everyday Americans in a country with growing income inequality.

Clinton appears unlikely to face a formidable Democratic opponent in the primary elections. Should she win the nomination, Clinton would face the winner of a crowded Republican primary field that could feature as many as two dozen candidates.

Kentucky Sen. Rand Paul, a favourite among libertarians, and Texas Sen. Ted Cruz, a champion of the right-wing tea party movement, have already entered the Republican race. Cuban-American Sen. Marco Rubio of Florida is expected to announce his bid to be the first Hispanic president on Monday.

Republicans were already trying to counter the former first lady's strong resume by casting her as someone who is not trustworthy. They have jumped on her use of a personal rather than a government email account and a server located in her home while she was President Barack Obama's first secretary of state. They have also raised questions about donations from foreign governments to the Clinton family's foundation.

Some Republicans sought to make foreign policy an issue at a time when the Obama administration is negotiating a nuclear deal with Iran and moving to restore diplomatic relations with Cuba.

"We must do better than the Obama-Clinton foreign policy that has damaged relationships with our allies and emboldened our enemies," said former Florida Gov. Jeb Bush in his own online video Sunday. Bush, the brother and son of former presidents is widely expected to join the race for the Republican nomination.

While there are serious policy differences among the dozen or so major Republicans considering a run for president, they appear to have all concluded there's little downside in starting early when it comes to going after Clinton -- an opponent that only one of them has the chance to potentially face.

The criticism came with added request for donations.

Bush hit up supporters with a fundraising appeal to help him stop Clinton's "liberal agenda." Paul started selling "Hillary's Hard Drive" on his website, a not-so-subtle reference to the email and server controversy.

Unlike eight years ago, when she ran and lost the Democratic nomination to Obama, Clinton and her personal history weren't the focus of the first message of her campaign Sunday. She made no mention of her time in the Senate and her four years as secretary of state, or her potential to make history as the nation's first female president.

Instead, the video is a collection of voters talking about their lives, their plans and aspirations for the future. Everyday Americans need a champion, and I want to be that champion," Clinton said.

Clinton hopes to avoid the same stumbles in 2008, when she entered the race as a heavy favourite only to be upset by Obama in Iowa.

Her first campaign event is Tuesday.

The road trip was Clinton's idea, aides said.

"When Hillary first told us that she was ready to hit the road for Iowa, we literally looked at her and said, 'Seriously?' And she said, 'Seriously,"' said longtime aide Huma Abedin in a conference call with Clinton alumni.

A Clinton aide said the van is nicknamed "Scooby" after the van in the 1970s animated television show, "The Scooby Doo Show."

The 67-year-old Clinton brings a long public record to her second bid for the White House, a history that will both help and hurt her candidacy. Republicans were already pushing a message that seeks to attach her to the scandalous upheavals of her husband Bill Clinton's two-term presidency in the 1990s.

Understanding that, her staff has said she intends to cast herself as a "tenacious fighter" determined to block the growing power of an increasingly right-wing Republican Party that has sought to block Obama's agenda and now controls both chambers of Congress.