On Thursday afternoon, a proposal to amend the US Constitution to allow tougher campaign finance and election spending restrictions was blocked in the Senate, in a party-line vote. 54 Democrats voted to advance the measure — another, Senator Kirsten Gillibrand (D-NY), would have done so but wasn't present. However, every single Republican voted against it, and it fell to a filibuster.
Unlike with other bills that have majority support, a filibuster wasn't the primary obstacle here. A proposed constitutional amendment has to win 67 votes to be passed by the Senate — so, assuming all Democrats were present, the amendment would still have been 12 votes short overall.
The amendment would have reversed not just Citizens United, but decades of Supreme Court holdings on political spending, going back to the 1970s. Supporters argued that such a measure is necessary to help reduce the unprecedented amounts of money being spent on elections, but opponents maintained it would hurt free speech and political participation. You can read Vox's full explainer on what the amendment would do here.