With some socialist and revolutionary parties boycotting the election to the First State Duma in February 1906, the Kadets received 37% of the urban vote and won over 30% of the seats in the Duma. They interpreted their electoral win as a mandate and allied with the left-leaning peasant Trudovik faction, forming a majority in the Duma. When their declaration of legislative intent was rejected by the government at the start of the parliamentary session in April, they adopted a radical oppositionist line, denouncing the government at every opportunity. On July 9, the government announced that the Duma was dysfunctional and dissolved it. In response, 120 Kadet and 80 Trudovik and Social Democrat deputies went to Vyborg (then a part of the autonomous Grand Duchy of Finland and thus beyond the reach of Russian police) and responded with the Vyborg Manifesto (or the "Vyborg Appeal"), written by Miliukov. In the manifesto, they called for passive resistance, non-payment of taxes and draft avoidance. The appeal failed to have an effect on the population at large and proved both ineffective and counterproductive, leading to a ban on its authors', including the entire Kadet leadership, participation in future Dumas.
It wasn't until later in 1906, with the revolution in retreat, that the Kadets abandoned revolutionary and republican aspirations and declared their support for a constitutional monarchy. The government, however, remained suspicious of the Kadets until the fall of the monarchy in 1917.
Finnish liberal politician and professor of jurisdiction and politology, Leo Mechelin, was expelled 1903-04, when the Kadets were preparing to form a party. Mechelin cooperated with them and wrote them a liberal constitution for Russia, to be enforced when they would get into power. At the time of Vyborg Manifesto, Mechelin was already the leader of the Finnish government ("Mechelin's senate", (1905-08)), which implemented the universal right to vote and freedoms of expression, press, congregation and association.
Due to the changes in the electoral law, the Kadets were reduced to a relatively small (54 seats) opposition group in the Third Duma (1907-1912). Although excluded from the more important Duma committees, the Kadets were not entirely powerless and could determine the outcome of certain votes when allied with the centrist Octoberist faction against right wing nationalist deputies. With the revolution crushed by 1908, they moderated their position even further, voted to denounce revolutionary violence, no longer sought confrontation with the government and concentrated on influencing legislation whenever possible. By 1909 Miliukov could claim that the Kadets were now "the opposition of His Majesty, not opposition to His Majesty", which caused only moderate dissent among the left-leaning faction of the party.
Although the Kadets, allied with the Progressive faction and the Octobrists, were able to push some liberal bills (religious freedoms, freedom of the press and of the labor unions) through the Duma, the bills were either diluted by the upper house of the parliament or vetoed by the Tsar. The failure of their legislative program further discredited the Kadets' strategy of peaceful change through gradual reform.
In 1910 the government rekindled its pre-revolutionary Russification campaign in an attempt to restrict minority rights, notably drastically curtailing Finland's autonomy. Most Kadets were opposed to these policies and, allied with the left wing of the Octobrists, tried to blunt them as much as possible, but were unsuccessful. However, a minority of Kadets headed by Pyotr Struve supported a moderate version of russification, which threatened to split the party. With the increase in popular discontent after the Lena massacre on April 4, 1912 and a continuous decline in party membership after 1906, the rift in the party became more pronounced. Kadet leaders on the Left like Central Committee member Nikolai Vissarionovich Nekrasov argued that the Duma experience had been a failure and that "constructive work" was pointless under an autocratic government. Kadet leaders on the Right like Central Committee members Vasily Maklakov, Mikhail Chelnokov, Nikolai Gredeskul and Ariadna Tyrkova-Williams, on the other hand, argued for a shift to the Right. The disagreements were temporarily put aside in July 1914 at the outbreak of World War I when the Kadets unconditionally supported the government and found an outlet for their energies in various kinds of relief work under the umbrella of the All-Russian Union of Zemstvos and the All-Russian Union of Cities.
Once the initial outburst of national unity feelings died down in mid-1915 as Russian retreat from Galicia showed the government's incompetence, the Kadets, together with the Progressive faction, the Octobrist faction and a part of the Nationalist faction in the Duma, formed the Progressive Bloc in August 1915, which was critical of the government's prosecution of the war and demanded a government of "popular confidence". As Russia's defeats in the war multiplied, the Kadets' opposition became more pronounced, culminating in Miliukov's speech in the Duma in October 1916, when he all but accused government ministers of treason.
The Kadets' position in the Provisional Government was compromised when Miliukov's promise to the Entente allies to continue the war (April 18) was made public on April 26, 1917. The resulting government crisis led to Miliukov's resignation and a powersharing agreement with moderate socialist parties on May 4-5. The Kadets' position was further eroded during the July crisis when they resigned from the government in protest against consessions to the Ukrainian independence movement. Although the coalition was reformed later in July under Alexander Kerensky and survived yet another government crisis in early September. Sergei Fedorovich Oldenburg was Minister of Education and served briefly as chair of the short-lived Commission on Nationality Affairs. The Kadets had become a liability for their socialist coalition partners and a lightning rod for Bolshevik propaganda. With the Bolshevik seizure of power on October 25-26, 1917, Kadet and other anti-Bolshevik newspapers were closed down and the party was suppressed by the new regime. Oldenburg along with a group of academics visited Lenin at the Smolny Institute to complain about the arrest of several fomer ministers of the Provisional Government.