Campaign clients have access to a 24-hour concierge service, staffed by veterans of President Obama’s campaigns, to handle last-minute media buys or reconcile election paperwork.

Because it knows their donor world, Amalgamated is willing to advance cash on new contributions, almost immediately. It works directly with “caging” companies that process campaigns’ direct-mail donations, making the money available in hours, rather than days or weeks. The bank’s wire room stays open until just before 6 p.m., hours later than most commercial banks, to enable cash transfers to TV stations.

That flexibility, Democrats said, helped allow their campaigns to more fully exploit the sophisticated targeting and data tools that have ramped up the tactical pace of election campaigns, in which the ability to execute hour-by-hour decisions is constrained only by cash flow.

“Two hours make a big difference,” said Rebecca Pearcey, the campaign manager for Ted Strickland, an Ohio Democrat seeking election to the Senate in 2016. “Especially if it is two weeks before the election, and you know that people in Akron are going to be watching ‘Wheel of Fortune’ on Thursday and you need to get in front of them.”

In some instances, Amalgamated has served as a kind of lender of last resort for campaigns and super PACs, organizations with little or no credit history and erratic cash flow. Ready for Hillary, for example, was founded two years ago and intended to stay in business only until Mrs. Clinton got into the 2016 presidential race, building lists of small donors and grass-roots organizers that could later be made available to her campaign.