ISTANBUL, Turkey — Turkey’s ruling party is likely to win a majority of seats in parliamentary elections Sunday that will highlight the deepening divide between the Islamic-oriented government and opponents who fear religion is encroaching on secular traditions.

While most voter surveys put the ruling Justice and Development Party well ahead of its rivals, it is expected to win fewer seats than in the 2002 elections and as a result could have less leverage when the new Parliament faces its first critical test: electing a president.

The vote by legislators for a new president could amount to a replay of a showdown in late April and early May when the ruling party’s candidate, a pious Muslim, was forced to abandon his bid after fierce opposition from the secular establishment. The military, perpetrator of coups in the past, threatened to intervene to safeguard secularism.

“The main issue is the presidential election,” analyst Taha Ozhan said of the campaign debate ahead of the parliamentary elections, which were called four months early to resolve the crisis. This time, the government has said it will seek a presidential candidate based on consensus, though a sense of uncertainty pervades Turkish politics.

On the streets, the mood is festive. Party flags and posters in Istanbul and other cities are plentiful, loudspeakers bark political slogans from roving vans and throngs of supporters with red-and-white Turkish flags cheer at campaign rallies.

Prime Minister Erdogan, who made predominantly Muslim Turkey’s bid to join the European Union a pillar of his agenda, has said he has no intention of imposing religion on politics. His government presided over reduced inflation, higher per capita income and more foreign investment, and enjoys the support of a growing class of conservative Muslims with economic and political clout.

Despite its achievements, the government faced a backlash when Foreign Minister Abdullah Gul tried to become president. Political opponents boycotted the parliamentary vote, arguing that Mr. Gul’s election to a position with veto power would remove the last obstacle to an Islamic takeover of the government.

Turkey’s top court, a secular institution, then declared the process invalid. At the time, huge crowds of pro-secular demonstrators rallied in major cities.

A research center in Ankara, the Foundation for Political, Economic and Social Research, assessed half a dozen polls by different companies and concluded that the ruling party would win between 38 and 41% of Sunday’s vote.

Under a law requiring parties to win 10% of the vote in order to enter Parliament, the ruling party would secure a majority of 290-310 seats in the 550-seat legislature, the center’s director of economic research, Ozhan, said.

Such a result would allow Mr. Erdogan to form a single-party government, avoiding the need for a coalition that would require compromises with political partners and could lead to deadlock. But it represents a reduction of seats from its current majority of two-thirds.

On Tuesday, Mr. Erdogan said he would quit politics after Sunday’s elections if his party failed to return to power with a majority that would allow him to rule alone and he challenged his opponents to do the same.

“If we can’t come to power alone, I’ll leave politics,” CNN-Turk television and others quoted Mr. Erdogan as saying.

According to surveys, the secular party that led the boycott against Mr. Gul’s presidential bid is expected to maintain its status as the second-biggest party in Parliament. The Republican People’s Party was founded by Mustafa Kemal Ataturk, the revered national founder who curbed Islamic influence after the fall of the Ottoman Empire amid the chaos of World War I.

A new factor is a nationalist party that appears poised to enter Parliament after a five-year absence, possibly by benefiting from perceptions that the government is soft on Kurdish rebels despite talk of a cross-border operation against their bases in Iraq.

If the Nationalist Action Party succeeds, its lawmakers could provide a swing vote in a presidential poll, though their choice would depend on a variety of factors, including the identity of the candidate.

Turkey, a NATO member, has warned it might go into Iraq to pursue rebels of the separatist Kurdistan Workers’ Party, or PKK, unless the United States cracks down on the PKK, deemed a terrorist organization by Washington.

“It has put Mr. Erdogan on the defensive as ‘weak on terror,’ complicating (the ruling party’s) strategy of running on its impressive record of reform and economic growth,” a former American ambassador to Turkey and a visiting fellow at the Brookings Institution, Mark Parris, wrote in an analysis.