(Bloomberg) -- The unofficial end of summer kicks off a more intense phase of a Democratic presidential race that has been led by moderate Joe Biden, with progressives Bernie Sanders and Elizabeth Warren in a battle for second as the rest of the sprawling field struggles to break through.“There’s a real sense that time is running out after Labor Day,” said Alex Conant, a public relations consultant who was a top aide on the 2016 presidential campaign of Senator Marco Rubio, a Florida Republican. “Voters become more engaged. Before Labor Day most voters are content to watch the show. After Labor Day they start picking a team.”He added, “Especially if you’re a second- or third-tier candidate who’s been waiting to make a move, the time is now.”Democrats have essentially culled the field to a four-person race with a few wild cards showing signs of life and others hanging by a thread. Party members are eager to nominate a candidate they believe can beat President Donald Trump, keeping Biden...