WASHINGTON — In a sign of an expanding alliance between the Trump administration and one of the most well-financed forces in conservative politics, Vice President Mike Pence has agreed to speak to a gathering of the billionaire Koch brothers’ advocacy network this month.

Mr. Pence will deliver the keynote address on Aug. 19 in Richmond, Va., to the annual meeting of activists and donors organized by Americans for Prosperity, the group announced on Friday. The nonprofit advocacy group is financed by the industrialist brothers Charles G. and David H. Koch, and their allies.

The Koch brothers steer a network of groups that spent between $720 million and $750 million to bolster conservative policy positions and politicians in the run-up to the 2016 election, according to people familiar with the spending. Yet the groups withheld support from President Trump’s campaign out of distaste for his populist and protectionist rhetoric, which clashed sharply with the Kochs’ support for free-market policies.

But in recent weeks, the Kochs’ groups — as well as other deep-pocketed conservative outfits — have expressed increasing support for Mr. Trump’s plans to overhaul the tax code.