A group of 10 Senators—five Democrats and five Republicans—last week announced they’ve agreed on the “framework” for such a deal. Details are scant, but leaks suggest the proposal would cost $1.2 trillion over eight years and include $579 billion in new spending. The deal is geared toward core infrastructure such as roads, bridges and broadband, though it would also include some of President

Biden’s

green-energy demands.

The Senators said in a statement that the deal would be “fully paid for and not include tax increases.” They are still haggling over “pay-fors”—with ideas that include indexing the gasoline tax to inflation, imposing electric-vehicle user fees, or creating a new federal infrastructure bank.

The country could use some infrastructure investment, and a focused bill that repurposes unspent Covid funds is the best option. The problem is what comes next. Republicans are uneasy about the size of the package. But they might be convinced to go along if they knew this compromise was the end of the Democratic spending plans. The White House would get its big infrastructure win, and Republicans would stop more left-wing damage.

But Mr. Biden has proposed trillions of dollars in additional spending, and progressives are already vowing to stuff anything that doesn’t make the infrastructure cut into a separate bill that could pass with only 50 Senate votes (plus the Vice President’s tie-breaker). Politico reports that Senate Budget Chairman

Bernie Sanders

isn’t “sweating” the bipartisan talks, because he’s already rallying support for his follow-on bonanza.

If an infrastructure compromise serves primarily to grease the skids for this phase two blowout, it will do far more harm than good. Two of the Democratic negotiators, West Virginia’s

Joe Manchin

and Arizona’s Kyrsten Sinema, continue to insist on bipartisan infrastructure talks, but they haven’t said what they’ll do on phase two.

The danger is that an infrastructure deal might make it easier to pass phase two. Democratic leaders face the prospect of selling a reconciliation bill that encompasses infrastructure, climate subsidies, and Mr. Biden’s “American Families Plan”—and could easily have a price tag north of $4 trillion. Carving off a trillion dollars or so with an initial infrastructure compromise would reduce that sticker shock, and arguably make it easier for swing-state Democrats to get on board.

This second bill would be the killer. Infrastructure spending may be wasteful or special-interest pork, but most of it will be a one-time event. Mr. Biden’s “families plan” is a cradle-to-grave expansion of entitlements that would rewrite the social contract: universal preschool, free community college, and new federal programs for child care and paid family leave. No work required. The entitlements would start small but become giant spending wedges in the future. The history of entitlements is that they are impossible to reform, much less kill.

GOP Senators have to decide if an infrastructure deal is worth the huge risk that it will pave the way for a phase two bill that expands the government by 4% as a share of GDP on a permanent basis. They also risk putting their fingerprints on a green-energy subsidy program that could become as politically embarrassing as

Barack Obama’s

2009 stimulus and its Solyndra scandals. The examples of crony socialism will be exquisite.

Republicans might not have to make this fateful choice, given that progressives are mobilizing to tank the deal. Democrats would need every member of their Senate caucus to join with 10 Republicans to overcome the 60-vote filibuster rule, but a half-dozen Senate liberals this week derided the bipartisan infrastructure framework as inadequate.

Perhaps that ought to be the GOP’s warning that Democrats intend to see their infrastructure compromise, and raise them a reconciliation spending free-for-all for the ages.