Mrs Clinton has received an endorsement from the New York Times
Democrat presidential hopeful Hillary Clinton faces her first big test tomorrow.
The rural state of Iowa is holding the first nominating contest - known as the caucuses - to pick candidates for November's election.
Both the Democrats and Republican caucuses in Iowa are being held tomorrow.
Yesterday, Mrs Clinton received an endorsement from one of the nation's largest newspapers - the New York Times.
The influential publication's editorial board wrote, "Hillary Clinton is the right choice for the Democrats to present a vision for America that is radically different from the one that leading Republican candidates offer".
The board also argued that Clinton's main rival, Bernie Sanders, "does not have the breadth of experience or policy ideas that Mrs Clinton offers. His boldest proposals... have earned him support among alienated middle-class voters and young people. But his plans for achieving them aren’t realistic, while Mrs Clinton has very good, and achievable, proposals in both areas".
An opinion poll from Des Moines Register/Bloomberg Politics has shown a narrow gap between the two candidates, with Mrs Clinton maintaining a lead at 45% to Sanders' 42%.
A second poll looking at the Republican candidates has again shown Donald Trump as the front-runner, with a 5% lead over Texas senator Ted Cruz.
In his report on the poll for Bloomberg, John McCormick writes, "a Trump victory could significantly boost his chances of winning his party's nomination, while a second-place finish for Cruz would be a major setback for a candidate who has invested heavily in Iowa and enjoyed strong support from evangelical Christians who form a large part of the state’s electorate".
Mr Trump pulled out of a televised debate with the other leading Republican candidates earlier this week, claiming he had been treated unfairly by host Fox News amid a months-long feud with anchor and debate moderator Megyn Kelly.
The billionaire businessman is said to have a significant lead ahead of the subsequent primaries in New Hampshire and South Carolina.