Top Democrats are preparing to make the case to impose new limits on the filibuster, in a move that could bring to a head six months of smoldering tensions over an expected Republican blockade of President Joe Biden’s legislative agenda. The Senate had its first filibuster of this Congress last week, when Republicans used the tactical rule to block a bipartisan House-passed measure to create a 9/11-style commission to investigate the Capitol attack perpetrated a pro-Trump mob.