The concern is that Italy's problems will spread and further unsettle other parts of the eurozone. Greece faces skepticism that it can keep paying its debts despite $310 billion in bailouts and Spain is still weighing whether to ask for a rescue from the eurozone's bailout fund.
Since it took office in November 2011 after markets lost confidence in Premier Silvio Berlusconi's half-hearted attempts at reform Monti's 13-month old government has managed to lower Italy's borrowing costs in the bond markets. His unelected cabinet of experts had until elections scheduled for April to implement reforms. Monti resigned earlier than expected after Berlusconi's party withdrew support for his government on Thursday.
Italy's borrowing costs started to rise again Monday morning as investors worried over who would keep the country on the path to recovery. The interest rate on Italy's 10-year bonds, a key indicator of the debt crisis, jumped to 4.84 percent Monday. Last week they yielded only 4.4 percent down from over 7 percent at the start of 2012. At one point during the day, Italian stocks slumped more than 3 percent.
The worry from Sunday's announcement is that heavily indebted Italy, the eurozone's third-largest economy after Germany and France, will now slow down or halt efforts to shake up its economy. The country's debt stands at 126 percent of its annual gross domestic product of $2.1 trillion.
After passing a 2013 budget, parliament will likely pass little in the way of new measures for several months. And Italy could come out of new elections expected in February with no party clearly in control. Meanwhile, the election campaign could see Berlusconi shake market confidence by campaigning on an anti-cuts platform.
"The uncertainty is significant, not just how the election will pan out, but how the government will turn out and what the debate is going to be like," said analyst Raoul Ruparel at the Open Europe think tank in London.
"And that's not something markets like. You add in the Greek uncertainty and Spanish uncertainty on top of that... and that's a worrying confluence of factors for a pretty fragile eurozone at the moment. "
Europe's crisis over too much debt was calmed after European Central Bank head Mario Draghi said in July that the bank would "do what it takes" to rescue the euro. He then followed up with a plan in September to buy unlimited amounts of government bonds issued by indebted countries, if they agree to reduce their deficits. The bond purchases would have to be preceded by a formal request for financial help from the European Stability Mechanism, the eurozone's bailout fund.
The purchases could push bond prices up and interest rates down, since prices and rates move in opposite directions. Just the possibility that the ECB might intervene had already sent borrowing costs down for Spain and Italy. Bond markets became much less threatening, even as Draghi and outside economists warned the bank could only buy time for countries to fix their deficits and reduce debt.
Monti's resignation raises the chance, analysts say, that the ECB will have to actually deploy its bond-purchase weapon. However, without a stable government that can credibly agree to the terms of a bailout, Italy can't get help from the ECB bond purchase plan. And the ECB plan is so far the main firewall holding the crisis at bay.
Holger Schmieding at Berenberg Bank said that the Italian election was one of those events "that could possibly spark some temporary market turmoil and trigger a Spanish (and perhaps even an Italian) request for ESM support and hence possible ECB bond purchases."
However, there are other analysts that predict the early Italian elections would not trigger severe market shocks.
First, markets knew Monti would be leaving soon anyway, with elections already scheduled in April. And if debt costs continue their rise, that could be the spur Italian politicians to move forward with pro-growth steps such as reducing legal protections against layoff for established workers. The practice deters hiring and raises youth unemployment. Monti made some changes, but was unable to push through wholesale reform in the face of union resistance.
Open Europe's Ruparel said that "not enough has been done" and that this kind of reform aimed at promoting improving the business climate and growth was in fact more crucial than reducing Italy's deficit.
Raj Badiani at IHS Global Insight said high bond market interest costs could "force the Italian political classes to refocus on the actions needed to push away intensifying" government debt pressures.
That's what happened in late 2011, when rising bond yields intensified fears Italy might default. Pressure from that helped force Berlusconi to resign and make way for Monti.
Markets are starting to ask if the next Italian government will face a similar crisis in 2013.