In the past two months, five House Democrats from competitive districts have announced they won’t seek re-election next year. They include Representative Charlie Crist of Florida, who on Tuesday kicked off a campaign for governor, and Representative Tim Ryan of Ohio, who will run for the Senate seat being vacated by Rob Portman. Three other Democrats will leave seats vacant in districts likely to see significant change once they are redrawn using the data from the 2020 census, and several more are weighing bids for higher office. An early trickle of retirements by House members in competitive districts is often the first sign of a coming political wave. In the 2018 cycle, 48 House Republicans didn’t seek re-election — and Democrats won 14 of those vacancies. Now Republicans are salivating over the prospect of reversing that dynamic and erasing the Democrats’ six-seat advantage.