(Reuters) - Facing a tough path to victory if they cannot win Ohio, Republicans Mitt Romney and Paul Ryan launched a two-day bus tour on Tuesday to try to boost their fortunes in a state that polls show could be slipping away from them.

The Republican nominees for president and vice president appeared together for the first time in more than three weeks, part of what aides vow will be a more aggressive phase of campaigning after Romney spent much of the past two weeks raising money and holding few public rallies.

Romney emphasized his claims that Democratic President Barack Obama's policies are preventing the U.S. economy from a full recovery and that Obama has not been tough enough in pushing back against Chinese trade practices that have led to cheap goods flooding the U.S. market and killing American jobs.

Romney accused China of a wide variety of trade abuses, from holding down the value of its currency to keep its products cheap to stealing U.S. intellectual property.

"We cannot compete with people that don't play fair, and I will not let that go on," Romney said.

Scrambling In Ohio

Romney's message on China has been a central part of his stump speech all year. Obama's campaign has responded with a television ad that accuses Romney of outsourcing jobs to China during his time as a private equity executive at Bain Capital.

Romney and his team hope the trade message will resonate in Ohio, a politically divided state that is key to his chances of winning the presidency but where polls indicate Obama has opened up a lead.

Ohio's unemployment rate in July was 7.2 percent, better than the national level of 8.1 percent.

It is difficult to see how Romney could win the Nov. 6 election without Ohio.

He has a narrower path than Obama to get the 270 electoral votes needed for victory, and most scenarios require him to win the Midwestern state where thousands of jobs were saved by the Obama-backed government bailout of the auto industry.

Romney aides dismissed surveys showing the Republican presidential ticket falling behind Obama in Ohio on a day in which a Washington Post poll showed the president with an 8-point lead over Romney, 52 to 44 percent.