Out of power, disorganized and stung by their November election losses, Democrats have struggled to mount a coherent response to President Trump as he has unleashed a blitz of contentious nominations, explosive executive orders and an unforeseen rampage by Elon Musk and his acolytes through the executive branch.

Now, with Congress returning from its spring recess on Monday and Republicans under pressure to deliver on a legislative agenda, Democrats believe they have a prime opportunity for a reset. Republicans will be forced to begin providing politically charged specifics of their tax and spending program, handing the minority ample fodder for attacks.

House Republicans have so far been able to generalize about their spending plans in the budget outline that they barely squeezed through the House and Senate. But beginning this week in multiple House committees, lawmakers will have to explain in public how they would achieve $1.5 trillion or more in savings to go with hundreds of billions of dollars in new spending on the military and border enforcement, and trillions in tax cuts.

“Up to this point, Republicans have been lying about their intentions,” Representative Hakeem Jeffries of New York, the House Democratic leader, said in an interview. “As we move forward with the next phase of the budget process, it will become crystal clear in committee after committee the damage they are trying to do to the American people.”

Democrats say that no cut is too small to highlight. They intend to highlight the consequences of the G.O.P. plan, from reducing food assistance for the poor to eliminating clean energy tax credits. They plan to call special attention to possible Medicaid cuts, an issue they have raised repeatedly in recent months.