The hearing underscores what has been a central dilemma for House Democrats all year — they have promised to investigate Trump, aggressively, and many of their base supporters want them to move quickly to try to remove him from office. But the White House has blocked their oversight requests at most every turn, declining to provide new documents or allow former aides to testify. The Republican Senate is certain to rebuff any House efforts to bring charges against the president. And moderate Democrats in their own caucus have expressed nervousness that the impeachment push could crowd out their other accomplishments.