"I think it's premature, though, to say that it's a foregone conclusion that President Saleh will not last, that the regime will not last," Boucek said.
Protests calling for Saleh's departure erupted in January and have intensified since mid-February.
The president, who has been in power since 1978 and is one of the region's great survivors, has said he will only step down after his term expires in 2013. But he has recently encountered some serious setbacks.
Anti-regime demonstrations have spread to the north, where supporters of the Shiite Huthi rebel group protested in the city of Saada on February 21 demanding Saleh's ouster.
Two days later, a member of Saleh's General People's Congress party said that he and seven other ruling party MPs had resigned to protest the repression of demonstrations, after two others had done so earlier in the week.
On February 26, the leaders of two of Yemen's most important and powerful tribes -- the Hashid and the Baqil -- abandoned the president and joined the popular uprising.
On Tuesday, leading Yemeni cleric Abdul Majeed al-Zendani appeared at an anti-Saleh demonstration, saying the protest movement sweeping the Arab world was "a new, effective, fast, and non-destructive means of changing regimes."
And on Wednesday, a top leader in Yemen's separatist Southern Movement urged southerners to avoid secessionist slogans and to throw in their lot with anti-regime protests, though a number of anti-regime demonstrations had already been held in the south.
But perhaps worst for Saleh, change itself has gained momentum after popular uprisings overthrew the veteran leaders of Tunisia and Egypt, sending shockwaves across the world and inspiring similar revolts against other Arab leaders, including Libya's Moamer Kadhafi.
"The street is creating a new momentum in Yemeni politics which is forcing a lot of key actors to reassess their alliances," said Ginny Hill, who leads the Chatham House Yemen Forum.
Ibrahim Sharqieh, the deputy director of the Brookings Doha Center, also said the tide has now turned against Saleh.
"There has been a serious shift in the balance of power in the past few days as a result of the tribal leaders joining the revolution," said Sharqieh.
"He lost significant power on the tribal level, which is a major front for him in his confrontation with the protesters," he said, adding that Zendani's backing of the protests was also significant.
However, Saleh "hasn't lost everything," as he is still "in control of some important institutions like the army," Sharqieh added.
Boucek also emphasised the importance of the Yemeni military, in which members of Saleh's family hold key positions.
"The military and the security services will be the key in how this moves forward, in the fact that they are all within the family, more or less," he said.
International backing for Saleh, Boucek said, will also be an important factor in determining whether Saleh stays or is forced out.
"Unlike Egypt or Tunisia or Libya, the other kind of cases that are going on, I think that Yemen still has the support of the international community, the United States, the Saudis and others," he said.
"As long as the Americans and the Saudis and the international community are still backing President Saleh, he's not going to leave."
Boucek pointed to international concern over Al-Qaeda in the Arabian Peninsula, the militant group's local franchise operating in Yemen, as a key factor in international support for Saleh.
Also, "the fact that no one knows what will happen after President Saleh is a huge concern that is driving people to stay with what they know instead of what they don't know," Boucek said.
"There is a growing sense that political unrest in Yemen is approaching a tipping point," said Hill, who also noted Saleh's importance to Washington.
"The US administration currently has all its eggs in one basket, with military aid going directly to elite security and intelligence units commanded by the president's son and his nephews," she said.
"If President Saleh loses control, this will have an immediate and direct impact on US counter-terrorism operations in Yemen," Hill said.
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