“What’s changed Susan?” reads a billboard truck parked outside the Capitol Building. Paid for by a liberal activist group, the billboard is just one more tactic the Democratic Party is using to put pressure on Maine Republican Susan Collins, one of the centrist senators who has indicated support for additional witness testimony during the Senate’s impeachment trial.

The Senate will begin weighing the charges against the president on Tuesday, but Senate Democrats have made it clear that President Trump isn’t the only one on trial: Collins is, too.