In 1936, Zhang Xueliang and Yang Hucheng kidnapped Chaing Kai-Shek in order to coerce him into forming a United Front with the CCP. After agreeing, Chiang returned to his post while Xueliang and Hucheng were punished for their actions.
If Chiang was unpopular enough for this plan to be successful, why couldn't the two seize power and declare a United Front themselves?
The key thing to point out here is that Zhang Xueliang kidnapped Chiang in December 1936 without being aware that he had already been in negotiations with Mao to declare a truce and prepare for conflict with Japan since the summer, when Stalin had urged the CCP to avoid destruction by cooperating with the KMT. (How far the CCP had been willing to acquiesce to Chiang's demands by early December is to some extent a matter of interpretation – Mitter, citing Taylor's 2009 biography of Chiang, argues that there was essentially a deal by the time of the kidnapping; van de Ven that Chiang's terms were still too severe as of late November for an agreement to be reached.) Having been largely cut out of the upper echelons of political power since his base region of Manchuria was occupied by Japan in 1931, Zhang was simply unaware of the secret discussions that had been ongoing between Chiang and Mao, and so the kidnapping actually did very little in terms of re-establishing the United Front. Moreover, Chiang was far from universally unpopular. There was indeed great indignation in the ROC about Zhang's apparent act of treachery, and he would remain under house arrest in Taiwan until 1990. Chiang was commonly recognised as probably the only major Nationalist Party member who could keep the conservative wing (e.g. T.V. Soong) and radical wing (e.g. Wang Jingwei) united, and so for the sake of national unity in the face of an imminent but uncertain war with Japan, his preservation was generally deemed essential – although at first there were some of Chiang's political enemies, particularly Mao, who did consider executing him while in captivity. Additionally, the CCP's involvement in the Xi'an Incident had occurred without the acquiescence of Stalin, who saw a KMT-CCP-USSR axis as necessary to defend against the new Anti-Comintern Pact of Germany and Japan, and so there was also outside pressure from the CCP's chief sponsors to prevent the ROC as a whole falling into disarray by removing its most capable leader.
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Was Young Hoo Chien killed by Chiang later? Why he did not give the same treatment to Zhang?