DES MOINES, Iowa — Bernie Sanders and Elizabeth Warren have spent the past year courting the Democratic Party’s base with appeals to the working class and sweeping promises to curb climate change. But as they balance their responsibilities to participate in the Senate’s impeachment trial and rally voters on the campaign trail, they’re turning to private air travel, an option typically reserved for the elite and criticized as environmentally unfriendly. Sanders is expected to charter a flight to Iowa this weekend while the Senate trial is in recess.