To summarise: Yes he had the opportunities to put himself in power, but he didn't use his opportunities well/at all and didn't see Stalin as a threat until it was too late.

To gain enough influence and support for him to take control of USSR he needed to have support from two groups of people. The government (so the Politburo, the Central Committee and the Party Congress) and the other group, were the general population of the USSR. Keep in mind that I am talking about the timeframe from about 1922-1929.

Trotsky was the most popular in the Red Army, due to his actions against the White Guard during the Russian Civil War. In the government Trotsky had a lot of opposition. His main opponents were three important members of government: Zinoviev, Kamenev and Stalin. This was called: the Triumvirate. They had the ability to outvote Trotsky in all of the three governing bodies I mentioned before. This made any actions of Trotsky to increase his support in the Politburo pointless. Example: October 1923, Trotsky criticized the Central Committee for their implementation of economic policy and also complained about the party in general. The Party Congress voted against his criticisms by a very large majority. Not only that but he was accused of factionalism even when he did manage to gather 45 members of the Party to sign and agree with his criticism.

Trotsky squandered his best opportunity with Lenin's Political Testament (finished early 1923), a document which criticized Stalin and recommending Trotsky as Lenin's successor.

"Comrade Stalin, having become Secretary-General, has unlimited authority concentrated in his hands, and I am not sure whether he will always be capable of using that authority with sufficient caution. Comrade Trotsky, on the other hand, as his struggles against the C.C. [Central Committee] on the question of the People's Commissariat for Communications has already proved, is distinguished not only by outstanding ability. He is personally perhaps the most capable man in the present C.C, but he has displayed excessive self-assurance and shown excessive preoccupation with the purely administrative side of the work." Lenin's Political Testament 1922

Trotsky chose to do very little with Lenins Political Testament. It also didn't help that both Zinoviev and Kamenev refused to publish the Testament and saved Stalin from the possibility of a demise. The Testament was discussed in the party in 1924, and it was decided that the final words of Lenin were to be kept secret from the general population. This was one of Trotsky's biggest mistakes, as this document provided him the opportunity to claim the country as Lenins official successor.

Trotsky also missed Lenin's funeral (21st Jan 1924). It may have been due to the fact that Trotsky became ill around this time, but there is also evidence suggesting that Stalin told Trotsky the wrong date. Missing Lenin's funeral damaged his Trotsky's reputation, especially in the eyes of the Soviet people. This also became an instrument of Stalins attack on Trotsky later on.

After the death of Lenin, Stalin and Trotsky engaged in writing political statements against each other. Stalin gained complete victory in the party. Trotsky lost most of his popularity by this point.

An important tool to Stalins rise and Trotsky's demise was Lenins death and the cult of personality and worship of Lenin created by Stalin. This allowed Stalin to leverage two things against Trotsky: The fact that Trotsky was a Menshevik allowed Stalin to denounce him to the party and to the people, and the fact that Trotsky missed the funeral, allowed Stalin to further denounce Trostky.

Trotsky was removed from the position of "Commissioner of War" in January 1925. He did not use his influential position in the Red Army to support himself.

Zinoviev and Kamenev started to be wary of Stalin. Even though they did everything they could have to stop Trotsky, in 1926 the three banded together in a movement called United Opposition. The movement failed as Zinoviev, Kamenev and Trotsky were removed form the party. All three were voted off the Politburo by the end of 1926, in October 1927 all three lost their positions in the Central Committee and were kicked out of the Communist Party. In 1928 Trotsky was exiled to Kazakhstan. At the start of 1929 he was expelled from USSR.

Trotsky posed the most threat to Stalin and his control of the USSR during the time of Lenins Testament. The threat to Stalin was immense, but the lack of support did not allow Trotsky to pursue his interests and the fact that he didn't use Lenin's Testament was to his detriment. And its not that Stalin planned everything perfectly, he famously called Lenins wife, Nadezhda Krupskaya:"a syphilictic whore", which could have also been his detriment. The fact that Zenoviev and Kamenev both supported Stalin until it was too late , definitely allowed him to gain the upper hand. And also the fact that Trotsky did very little with his influence in the Red Army did not benefit him at all.

Oxley, Peter. Russia: 1855 - 1991 ; from Tsars to Commissars. Oxford: Oxford U, 2001. Print.

Morris, Terry, and Derrick Murphy. Europe: 1870-1991. London: Collins Educational, 2000. Print.

McNeal, Robert H. “Lenin's Attack on Stalin: Review and Reappraisal.” American Slavic and East European Review, vol. 18, no. 3, 1959, pp. 295–314. www.jstor.org/stable/3004128.

"Lenin's Political Testament." Historyguide.org. N.p., n.d. Web. 01 Jan. 2017.

[Can provide further material if asked, these were the ones that were on hand]

Edit: Spelling/grammer, Russian Civil War instead of Revolution

Follow up question: did Trotsky pose any threat at the time of his assassination (or even believed to pose a threat)?