As one of the country’s largest and oldest conservative advocacy groups, the American Conservative Union has long fought to rein in federal spending and limit the size of government.

But behind the scenes, the group has formed a partnership with business lobbyists to tame the activists who have pushed Republican leaders in Congress to adopt some of the most austere spending limits in decades.

In a draft proposal circulated to defense and transportation industry executives in recent weeks, the union is offering to use its grass-roots organization, annual conference and movement clout to lobby against cuts to federal military and infrastructure spending.

The group is also proposing to incorporate favorable votes on military and infrastructure spending into its widely cited Congressional voting scorecard, “the ‘gold standard’ for elected officials,” according to the proposal, a copy of which was obtained by The New York Times. The documents shed light on a rarely public corner of Washington lobbying, where industry lobbyists join with grass-roots groups that offer ideological credibility and deep mailing lists of sympathetic activists — sometimes for a price.