By MATT BRADLEY

CAIRO—The runaway victory that Egyptian voters handed to Islamist parties in recent parliamentary elections is looking increasingly Pyrrhic.

The Muslim Brotherhood and the Nour Party, which identifies with the hard-line Salafi school of Islam, captured more than two-thirds of the seats in Egypt’s Parliament. But in the three months since, they have been largely ineffective. In recent weeks, the parties have faced mounting public criticism, internal defections and weakening prospects in next month’s presidential vote.

While Egyptian political polls are subject to broad skepticism, one conducted late last month indicates how far Islamist politicians’ public star has fallen: The Al Ahram Center for Political and Strategic Studies, a government-owned think tank, found that 45% of people who voted for the Brotherhood’s Freedom and Justice Party in the parliamentary elections said they wouldn’t do so again. The FJP holds nearly 50% of the seats in Parliament.

Egyptian Islamists blame their fading fortunes on unreasonably high public expectations and what they say is a political system that is rigged against them. While the Islamists hold a broad majority of Parliament, they don’t control Egypt’s cabinet, as would generally be the case in parliamentary systems.

Brotherhood officials say the council of generals who are overseeing Egypt’s political transition have blocked parliamentary efforts to dismiss the military-appointed cabinet of ministers. The interim regime has also deliberately blocked legislation originating in the Parliament, they say, in what they call a bid to paint the Islamists as political failures.

The government hasn’t publicly responded. Former Gen. Sameh Saif al-Yazal, who regularly consults with the leading generals, said that appointing a placeholder cabinet just a few months before Egypt’s scheduled transfer of power would be destabilizing.

Even weakened, Egypt’s Islamist parties remain a potent political force. The ground-level outreach of the Brotherhood and Salafis remains unrivaled by more liberal politicians, who are divided and unpopular.

The Islamists’ organized presence throughout Egypt helped them in parliamentary elections, where many voters, facing slates of largely unknown candidates, cast ballots based on familiar party names. The May 23 presidential polls, however, will hinge far more on candidates’ personalities than their party affiliations, political analysts say.

That could benefit more moderate, charismatic presidential candidates such as Amr Moussa, the former secretary-general of the Arab League, and Abdel Moneim Aboul Fotouh, a relatively liberal ex-Brotherhood leader.

Mr. Fotouh split with the organization last summer in a disagreement over the group’s promise not to nominate anyone for the presidency. The Brotherhood’s subsequent decision to run two presidential candidates has angered many voters, who saw the move as a power play.

Brotherhood officials say they decided to enter the race to fight what they call a calculated attempt by the military to strip the Parliament of political power.

“Because Parliament can’t do as much as people originally thought, the presidency has become much more of a prize,” said Shadi Hamid, the director of research at the Brookings Doha Center in Doha, Qatar.

That prize won’t go to the Brotherhood’s highest-profile candidate. Last week, an Egyptian judicial commission disqualified several candidates including Khairat al-Shater, a prominent financier and strategist within the Brotherhood organization who was the group’s first choice for president. Mr. Shater was excluded because of a prior fraud conviction, leaving the Brotherhood to lend its name and now-declining public stature to a lesser-known backup candidate, FJP Chairman Mohamed Morsi.

On Thursday, Egypt’s election commission released its final list of 13 candidates. The relative support levels for the remaining candidates remain unclear.

Egypt’s Islamists say their troubles are based in part on a constitutional declaration passed last year, with Islamist parties’ backing, that left most political authority with the council of military generals that took power from Hosni Mubarak when he stepped down from the presidency last year.

“It’s not an executive authority. It’s just a legislative authority with no power of implementing these laws and this legislation,” said Amr Al Makki, a Nour Party spokesman. “So the people, if they do not see any change or any implementation, they will not be happy.”

Even among Egypt’s profoundly religious electorate, voters appear to be holding religious parties responsible for accomplishing practical, worldly results to match their lofty religious rhetoric.

“The expectations are higher because they come from a religious background,” said Omar Ashour, the director of Middle East Studies at the University of Exeter, of the Islamist parties in Egypt’s parliament.

Similar dynamics are playing out in other Arab countries where a wave of successive pro-democracy uprisings last year yielded unprecedented victories for Islamist parties.

In Tunisia, the Al Nahda Party has recently faced criticism over its inability to rein in unemployment and inflation. Recent polls have shown declining public support for the party, which won about 40% of elections in October. In Libya, the role of religion in politics has also come to the fore: Religious parties have decried an apparent effort Wednesday by the country’s ruling National Transitional Council to ban religious-based parties.

Egypt’s parliament, meanwhile, stepped up its battle with the military on Tuesday, when it voted to reject an economic reform plan drafted by the military-appointed government—a move that could jeopardize negotiations for a $3.2 billion loan from the International Monetary Fund that is crucial to rescuing Egypt’s flagging economy.

Such a lack of effectiveness has led to disillusionment and infighting among lawmakers, some of whom appear to want to dissociate themselves from public scorn.

At least five parliamentarians resigned from the Salafi Nour Party recently, according to local Egyptian media. The party, which unlike the Brotherhood is not bound by a strict heirarchy or a unified ideology, has had difficulty maintaining internal discipline. While those who resigned cited multiple reasons, some said they were ashamed of the Islamists’ early political failures following their triumphant rise to power during the parliamentary elections.

“The performance of the FJP and Nour party was not living up to the magnitude of the revolution,” said Nezar Ghourab, a former Nour Party member from Giza, a Cairo suburb, who resigned this week.