Ohio vote-counting machines – brought to you by the Romneys

Barack Obama's two-day campaign blitz travels through eight states with stops in Iowa, Colorado, Nevada, Ohio and Virginia (AP)

Mitt Romney's presidential campaign has been hit by accusations about his links to a firm which provides the vote-counting machines that will be used in the crucial state of Ohio, as the tightly fought race increases concerns about voter fraud.

Ohio is seen as the closest and most important state in the election. The close contest between Mr Romney and President Obama could easily come down to the count there. Mr Obama still has a narrow lead over the Republican challenger in Ohio according to polls.

But Salon magazine has reported on the "extensive corporate ties to the Mitt Romney camp" of Hart InterCivic, which provides the vote-counting machines that will be used in Ohio, and the other key swing states of Colorado and Virginia. Reports in the US media since the end of last month have linked the company to Mitt Romney's son Tagg and to Romney campaign donors.

The Washington Post noted that the implication is that: "Romney will enjoy some kind of malign leverage over the vote count in Ohio." Hart InterCivic said it had a "long track record of supporting a fair and open democratic process. Any suggestions the company might try to influence the outcome of election results is unfounded."

The controversy has provided a timely reminder that how the votes are counted could partly decide the winner of the 6 November election.

In 2004, the role of vote-counting machines came under scrutiny in Ohio, where George W Bush defeated Democratic challenger John Kerry by 118,601 votes out of the 5.6 million cast.

The result was controversial because Walden O'Dell, the chief executive of an Ohio-based vote-counting machine manufacturer, had earlier declared his commitment to helping secure Mr Bush's re-election.

Barack Obama told crowds at back-to-back rallies across the US that the contest for the White House comes down to trust. "You know me. You know I say what I mean, and I mean what I say," he declared in Iowa.