Afghan presidential candidate Abdullah Abdullah, center, with presidential candidate Zalmay Rassoul, left, and presidential candidate Gul Agha Sherzai attend a joint news conference in Kabul. (Omar Sobhani/Reuters)

The front-runner in Afghanistan’s presidential contest picked up a major endorsement Sunday as he sought to consolidate his support ahead of an expected runoff next month.

Abdullah Abdullah, a former foreign minister who was the top vote-getter in the April 5 election, appeared in Kabul with the third-place finisher, Zalmay Rassoul.

Rassoul, who was thought to be outgoing President Hamid Karzai’s favored candidate, told reporters he was supporting Abdullah to send a message to Afghans that it was time to unite behind one candidate.

“I want to address all Afghans from this tribunal to vote for Dr. Abdullah to strengthen national unity and to encourage the political stabilization of this country,” said Rassoul, who is also a former foreign minister.

In the country’s third presidential election since the U.S.-backed invasion in 2001, Afghans flocked to the polls last month despite repeated threats of violence from Taliban militants.

According to preliminary returns, Abdullah received 44.9 percent of the vote compared with former finance minister Ashraf Ghani’s 31.5 percent and Rassoul’s 11.5 percent. Only the top two vote-getters will move on to the runoff.

Both Abdullah and Ghani are viewed as moderates with pro-Western views. They both have vowed to combat corruption and say they will sign a long-term security agreement that would permit a residual force of American troops to remain in Afghanistan after this year.

But some political leaders and analysts fear that a runoff could inflame ethnic tensions and complicate the transition between Karzai and his successor.

Abdullah’s father was Pashtun, but he has been more closely associated with his mother’s Tajik heritage. Ghani is Pashtun and is hoping to rally Pashtuns around his candidacy. Pashtuns are Afghanistan’s largest ethnic bloc, and Abdullah could be the country’s first ethnic Tajik leader since 1996.

On Sunday, Abdullah used Rassoul’s endorsement to make the case that he would be an inclusive leader.

“Today, we are gathered here because of the proximity of our visions as well as the closeness of our” policies, Abdullah said.

Rassoul is Pashtun and resides in southern Kandahar province. Gul Agha Sherzai, a Pashtun presidential candidate who endorsed Abdullah two weeks ago, also appeared at Sunday’s event.

In recent days, some Abdullah advisers have suggested that Rassoul’s support for the front-runner could persuade Ghani to drop out of the race. Abdullah is increasingly adopting a posture of invincibility, saying in a recent interview that he is all but certain to be Afghanistan’s next president.

But Hamidullah Farooqi, a senior adviser to Ghani, played down the significance of Rassoul’s endorsement. He noted that other members of Rassoul’s campaign team, including his vice presidential candidate, did not join him in backing Abdullah.

“Dr. Rassoul, we know, does not have much impact on the grass roots,” Farooqi said.

Farooqi predicted that Rassoul’s strategy of trying to build a broad coalition would backfire with voters. He noted that past governments that had included a mix of ethnic and political leaders had been troubled by infighting and corruption.