By Joseph Ax (Reuters) -The Democratic Party’s congressional campaign arm is airing an ad in Michigan aimed at boosting John Gibbs, a far-right candidate endorsed by Donald Trump, as he challenges one of the few Republicans who voted to impeach the former president. The effort is part of a controversial and risky Democratic strategy in certain races to elevate more extreme Republicans whom Democrats view as easier to beat in November’s midterm elections. Democrats have spent millions on candidates who echo Trump’s false claims that the 2020 election was rigged, even as party leaders say those …