By Jason Lange and Alexandra Ulmer WASHINGTON (Reuters) – Former President Donald Trump’s online fundraising has slowed in recent months, a financial disclosure on Friday showed, adding to doubts over the firmness of his grip on the Republican Party. Trump’s political committees raised about $18 million between April and June, about $2 million less than in the prior three months, according to a report by WinRed, the Republican donation processing portal. WinRed’s latest reports have shown Trump’s money-raising falling in seven of the last nine months, with tiny gains in May and June. Trump’s d…