DES MOINES, United States (AFP) - The Des Moines Register, the biggest newspaper in Iowa, on Saturday endorsed Hillary Clinton and Marco Rubio, just days before the US state kicks off the American presidential nominations process.

The Iowa caucuses, to be held on Feb 1, are seen as the first big test in the marathon to the Nov 8 election, and both Democratic and Republican contenders have been courting voters in the hopes of coming out ahead.

In endorsing Democratic frontrunner Clinton over rivals Bernie Sanders and Martin O'Malley, the paper's editorial board said the former secretary of state was "not a perfect candidate." But it concluded that "Clinton has demonstrated that she is a thoughtful, hardworking public servant who has earned the respect of leaders at home and abroad."

"She stands ready to take on the most demanding job in the world," it said of the 68-year-old former first lady.

"In the final analysis, Iowa Democrats will have to choose between the lofty idealism of Bernie Sanders and the down-to-earth pragmatism of Hillary Clinton," the board said.

"For some, this will be a choice of whether to vote with their hearts or their heads." On the Republican side, the newspaper referred to Rubio as a "whip-smart senator" who could take the GOP down a new path.

"Sen. Marco Rubio has the potential to chart a new direction for the party, and perhaps the nation, with his message of restoring the American dream," the editorial board said.

"We endorse him because he represents his party's best hope."

The 44-year-old Cuban-American from Florida is part of an unusually large and fractured Republican field led by billionaire Donald Trump.

The editorial board said it also valued the "executive experience, pragmatism and thoughtful policies" of Rubio rivals John Kasich, Chris Christie and Jeb Bush and that Rubio had much to prove.

"Yet most Republicans aren't interested in rewarding a long resume this year," it said. "They want new and different."