“We blame the security forces because of their negligence in protecting our candidates,” said Rahim al-Shimari, a spokesman for Iraqiya. “In spite of the repeated targeting, we have not seen any precautionary measures taken to limit the attacks.”

In offering theories about the attackers, experts and Iraqi officials say that in addition to the political rivals who are thought to be behind some killings, the militant group Al Qaeda in Mesopotamia is believed to be responsible for others. The group, which is also known as Al Qaeda in Iraq, targets Iraqi Shiites, but also Sunnis who accept the Shiite-led government or even just participate in politics, declaring them apostates.

“Although it is possible that some of the attacks may be the result of political rivalries, many of the attacks bear the hallmark of Al Qaeda in Iraq,” Ahmed Ali and Stephen Wicken, analysts for the Institute for the Study of War in Washington, wrote in a recent online column. “Historically it has opposed Sunni Arab participation in the political process.”

The killings have added to the sense of embattlement by the country’s Sunni Muslims, who are a minority here and who have been holding protests against the Shiite-dominated government. In Anbar and Nineveh Provinces, where Sunni-led protests have increased in recent months and where most of the assassinations have occurred, the government has postponed elections for a month, citing security concerns. They did so despite objections from local leaders, the Iraqi election commission, the United Nations and American diplomats.

“I have expressed my concern about this decision, as the citizens of these provinces are looking forward to these elections with great hope,” Martin Kobler, the United Nations representative in Iraq, said in recent testimony to the Security Council in New York.

On a visit to Baghdad, Secretary of State John Kerry criticized the decision and asked Prime Minister Nuri Kamal al-Maliki to reconsider. Mr. Maliki only reiterated the decision, demonstrating the limits of American influence over Iraq’s affairs.

The election postponement, despite the violence, is largely seen as a political move by Mr. Maliki to prevent the election of Sunni candidates who are opposed to the central government. At least that is what Iraqiya leaders believe. After all, they say, every election since 2003 has been held amid the threat of violence.