President Bush made an unusual visit to Capitol Hill last week to support the little-loved “grand bargain” on immigration. Most objection in the Senate has come from fellow Republicans who regard instant amnesty for 12 million or more illegal immigrants with a wary eye.

Let us throw out a few considerations. The first is that there’s been too little information on the costs to taxpayers of legalizing many millions of mostly unskilled workers, who would instantly be eligible for a variety of public benefits.

For example, how would the plan affect spending on the Earned Income Tax Credit? This is a federal program that returns money to low-income workers, in effect subsidizing their pay. It is a worthy program, but one that would be strained if huge numbers of legalized people suddenly qualify for it. The Congressional Budget Office estimated that the credit would cost U.S. taxpayers $20 billion in the first 10 years after the proposed amnesty went into effect.

The Heritage Foundation has come up with a projected cost of retirement benefits for this group, including Medicare, of $2.6 trillion. We can’t assess the accuracy of that number, but it is a large one. Let’s hear other projections.

Mr. Bush had been marketing an amnesty directed toward people who have been in America a long time and pay their back taxes. But the bill sets the deadline date at Jan. 1 of this year — less than seven months ago — and although it calls for $5,000 fines, it totally drops the requirement for paying back taxes.

We are not averse to the idea of an amnesty, but it must be paired with a genuine commitment to stopping future illegal immigration. That was to be the deal on the Immigration Reform and Control Act of 1986. The amnesty came off without a hitch, but the sanctions against the employers who hire illegal workers were ignored. Instead of requiring firms to check an applicant’s eligibility against a national database, the law let them simply accept identification documents that looked plausible.

A huge market in counterfeit and stolen identities bloomed, in effect gutting the law. Making matters worse, President Bush virtually abandoned all efforts to go after employers who blatantly employed illegal workers. The new bill is supposed to overcome that flaw — it does require that IDs be checked against a computerized registry — but given the pathetic history of early immigration-policy “reform” and the opposition of low-paying employers to these new rules, we can’t help but be skeptical.

Wouldn’t it make sense to boost everyone’s confidence by first enforcing the law on the books? Add to that a database requirement and let’s see what happens.

America already admits legally about three-quarter of a million immigrants every year. We question adding so-called temporary workers to the total. First, we would fully expect large numbers of these “temporary” guest workers to simply overstay their visas, as happens now. (Who can blame them?) So the temporary-worker program must realistically be regarded as adding to the permanent numbers.

We are also troubled by the notion of an indentured-servant-like class of worker. Temporary workers can be exploited by their employers and they’d further depress the wages of unskilled Americans and legal immigrants.

Americans need a lot more serious analysis of the grand bargain before they sign on. It may not be much of a bargain at all.