It was widely assumed ahead of the vote that the center-left would join forces with Monti's bloc, but its disappointing showing means that even together, they cannot command a majority in the upper house.
Bersani held a clear lead in polling in the weeks ahead of the vote but saw it slip away in the final days of the campaign.
Walston, the analyst, said ahead of the vote that the left had suffered from a lack of leadership, which prevented it from putting forward a clear picture. Bersani, 61, comes across as "bluff and homespun, and that's part of his appeal -- or not, depending on your point of view," said Walston.
The center-right alliance led by Berlusconi's People of Freedom Party, or PdL, in coalition with the right-wing, anti-immigration Northern League, made up significant ground in the final days of campaigning.
Berlusconi gave conflicting signals as to whether he was running for the premiership, indicating that he would seek the job if his coalition won, but contradicting that on other occasions.
In a phone interview Tuesday with Italian journalist and TV host Maurizio Belpietro, he said everyone had tried their best to help their country.
"The difference this time is that we weren't able to see through to the end and win," Berlusconi said.
He acknowledged that the confused result has done nothing to help market confidence.
But, he added, "I don't think Monti will be able to form any type of coalition with his policy of austerity and his dangerous economic policies that have hurt Italy."
Unless either Bersani or -- even less likely -- Berlusconi can persuade Grillo to enter an alliance, their options are limited.
Grillo, the 64-year-old known as the "clown prince" of Italian politics, has been outspoken in his criticism of Berlusconi and other politicians in the three years since he founded his party, known as M5S.
CNN iReporter Elisa Bozzi, an Italian art curator, said she was very disappointed by the outcome of the election.
"Yesterday when I saw the result I was completely without words," she said. "I think we in Italy will go again to vote because it's impossible to govern a state in this situation, (where) we don't have a majority."
She believes many voters hold Monti responsible for Italy's current debt crisis without remembering the dire economic situation he inherited when Berlusconi resigned in 2011. At the same time, Bersani failed to gain people's confidence, she said.
"Beppe Grillo has many votes because people are angry and frustrated," Bozzi said. "People in Italy are completely disappointed by the situation we see day-by-day, by corruption, scandals."
Such is voters' disillusion that Bozzi believes Grillo could garner enough votes in fresh elections to win outright.
Berlusconi, the septuagenarian playboy billionaire nicknamed "Il Cavaliere," campaigned as a Milan court weighed his appeal against a tax fraud conviction, for which he was sentenced to four years in jail last year.
The verdict has still to be delivered; however, under the Italian legal system, he is entitled to a further appeal in a higher court.
Because the case dates to July 2006, the statute of limitations will expire this year, meaning there is a good chance that none of the defendants will serve any prison time.