Greece's outgoing prime minister, Alexis Tsipras, is banking on his popularity to win a national election next month and strengthen his grip on power after purging his radical left Syriza party of dissenters.

But as the political jostling heats up ahead of the Sept. 20 vote, it appears increasingly likely that Tsipras will have to form a new, more unwieldy coalition government — possibly with as many as three other parties.

Tsipras called the election last week to rid his party of rebels who had refused to back the bailout deal he agreed upon with Greece's international creditors. The hardliners accused Tsipras of reneging on their party's promises to fight the budget cuts and tax increases demanded by creditors. Their defections came thick and fast, trimming Syriza's presence in parliament.

Now, as Tsipras weighs his options, experts say he may have to strike an alliance with the opposition, creating a so-called "grand coalition."

"The most likely outcome is a coalition government with two, three or four parties taking part," said Theodore Couloumbis, a political scientist at the University of Athens.

Tsipras and his supporters say they are aiming to capture an outright majority of seats in the new parliament, hoping that the new Syriza, shorn of its hardliners, will attract enough moderate voters to more than make up for the defections.

This will be difficult to achieve, experts say.

The first major opinion poll since elections were called, published Friday in the left-leaning Efimerida ton Syntakton newspaper, showed Syriza as the most popular party, with 23 percent saying they intend to vote for it. That was down from 26 percent in early July.

The second-biggest party, the conservative New Democracy, appears to be catching up, with 19.5 percent of the intended vote, up from 15 percent in July.

The poll was conducted by the ProRata company on Aug. 25-26 with a sample of 1,000 people nationwide and had a margin of error of 3 percentage points.

Much will depend on Tsipras, who is personally quite popular. Greeks feel that, although he agreed to the tough budget austerity measures in the new bailout, he did what he could to convince creditors to ease the terms. He also enjoys an image as a down-to-earth politician who dismantled the old, established parties that led the country into financial ruin six years ago.

"He is still seen as the fresh face, the likable David taking on the Goliaths," writes Aristides Hatzis, a professor of Law and Economics at the University of Athens Law School.

A Syriza majority would give Tsipras a strong mandate to implement the terms of the bailout — a three-year plan of budget cuts, tax increases and broad economic reforms in return for 86 billion euros ($97 billion) in loans from Greece's European partners.

Critics, however, note that as the austerity measures bite and create discontent among Greeks, more lawmakers in Syriza might be tempted to defect. Many of them, including Tsipras, note that the policies go against their beliefs and they are supporting them only because the alternative to a bailout would have been to let Greece fall out of the shared euro currency, something most Greeks do not want.

Short of a majority, Tsipras would first look to renew Syriza's coalition with the Independent Greeks, a small right-wing party that had quietly backed all his policies. But in the ProRata poll, only 2 percent said they would support the Independent Greeks, below the 3 percent needed to enter Parliament.

If the Independent Greeks cannot guarantee Syriza a majority, things get more complicated.

Syriza would almost surely reject the idea of an alliance with the Popular Unity, the new party formed by its own dissidents.

So it would be forced to turn to the old established parties: the center-left To Potami or the socialist PASOK, or even the center-right New Democracy.

The problem with this option is that Tsipras accuses them of being corrupt and allied with business interests. Tsipras said this week that if an alliance with these parties is the only option, he would not accept being prime minister. That has been widely interpreted as meaning he would let someone else take the role, while leading his party within the coalition government.

That would raise questions on who the new government leader would be — and whether they would be as dedicated to implementing Greece's bailout program as Tsipras.

All this coalition talk is predicated on one, rather shaky, assumption: that Tsipras is, or has become, a sincere reformer who has purposely rid himself of his left-wingers and is now ready to transform his neo-communist party into a social-democratic one. Some analysts have even called him, long before he signed the new bailout, an American-style liberal.

Beyond the governing coalition, the composition of the new parliament will also be crucial to the success of the bailout program.

A strengthening of the anti-bailout forces, such as the Popular Unity party, the Communist Party or the extreme-right Golden Dawn, might cause Syriza to gradually backslide from its pro-reform position and start dithering on reforms.

And that, in turn, could lead to a new clash and crisis with Greece's European creditors.