The European Central Bank’s newly announced bond-buying program continued pushing regional stock markets up and key interest rates down Friday – just the sort of confidence-building effect that its president, Mario Draghi, had hoped for.

But in the long term, the effectiveness of the the program unveiled Thursday remains in doubt.

Will the ECB initiative work as well as emergency efforts by the Federal Reserve during the U.S. financial crisis, when the Fed’s willingness to buy billions of dollars of troubled securities convinced private investors to return to the credit markets?

Or will the ECB program meet the same fate as an earlier European effort, before the euro zone was established, to keep the region’s exchange rates in line with each other? That program was abandoned after speculators overwhelmed Europe’s central bankers.

The first test is likely to come from Spain. The country is riding a fine line between needing the sort of bailout provided to Greece, Ireland and Portugal by international lenders and retaining enough trust among investors that they keep lending Spain money.

Spain doesn’t want to ask for a bailout. But to get the ECB’s help, the nation will have to make a formal appeal to the rest of the euro zone for help and agree on a series of economic reforms.

Spanish officials have been adamant that they can put a successful deficit-cutting and economic reform program in place without bailout loans or the outside European or International Monetary Fund oversight that would go along with the emergency assistance.

Spanish officials have yet to say whether they will avail themselves of the ECB’s help. “These matters must be analyzed calmly and carefully,” Deputy Prime Minister Soraya Saenz de Santamaria told journalists in Madrid on Friday, according to wire service reports. “They have important implications for our country and our future.”

Spain is already relying on European bailout funds to pay for an expensive rescue of the country’s banking system. Analysts expect it is only a matter of weeks before the government in Madrid requests broader assistance.

The promise of unlimited bond purchases and, implicitly, a cap on Spain’s high borrowing costs “has now thrown down the gauntlet to the government of Spain,” analysts from Barclays wrote Friday. “The government of Spain has little choice” but to respond.

The rescues of Greece, Ireland and Portugal involved large loans from the rest of Europe and the IMF. The help allowed the countries to stop selling government bonds for several years — in essence insulating them from the international markets while repairing their finances.

For Spain and perhaps Italy, the aim is to ensure that market access is never lost. Europe and the IMF would be hard-pressed to pay the debts of these two major economies. And a default in either country could damage the world economy.

Instead, Spain may ask to be approved for a precautionary credit line — a program that would give the country access to emergency funds if it needs them. To qualify, Spain would have to vet its finances and budgets with the IMF and other European nations. If the credit line were approved, the ECB would be willing to open the taps and buy bonds from existing investors. By boosting the demand for government bonds, the ECB would aim to keep the interest rates at a reasonable level so the country can continue to borrow the money it needs and never have to use the emergency funds.

Europe’s bailout fund, the European Stability Mechanism, could also be involved in buying longer-term bonds directly from the government — a further way for Europe to help Spain and perhaps Italy.

The program has raised objections from some, particularly in Germany, who are worried it will concentrate the risks of any government default within the ECB and the bailout fund — essentially making it the responsibility of European taxpayers rather than investors who have taken a willing gamble.

Draghi has been quizzed about other risks as well, such as that governments might start selling only three-year bonds in hopes of goading the ECB into buying more, or that speculative attacks might test his willingness to protect the currency. Draghi has said he would “do whatever it takes.”

Stressed by such attacks in the early 1990s, the Bank of England eventually abandoned efforts to keep the pound in line with the German mark — the effective end of a currency management system that predated the euro.

Draghi, noting that the ECB’s asset holdings are equal to only about 3 percent of the region’s economic output — less than that accumulated by the Fed and the Bank of England — said the bank has plenty of firepower and the will to use it.

“We say the euro is irreversible. So unfounded fears of reversibility are just what they are,” Draghi said, “unfounded fears.”