PRAGUE (Reuters) - The main Czech center-left party's leadership agreed on Tuesday to back a leftist government formed by allies of President Milos Zeman, improving its scant chances of winning a confidence vote on Wednesday, a senior party leader told Reuters.

The Social Democrat party overcame its initial resistance to a government imposed by the president against the will of the main political parties, which many politicians see as a power grab infringing on parliament's powers.

Prime Minister Jiri Rusnok, a longtime Zeman ally without his own political party, is still likely to lose the vote on Wednesday if center-right parties stick to their pledge to remain united in their stance against the government.

Losing the confidence vote would force Rusnok to resign, prolonging political instability that started with the resignation of the previous center-right cabinet in June. Czech remains mired in a recession and the political standoff casts doubt over the 2014 budget.

The party backed Rusnok because of fears that his failure could open the way for the former center-right coalition to return to power if Zeman gave it the next attempt to form the cabinet.

The Social Democrats' decision is a defeat for Chairman Bohuslav Sobotka, who has opposed Rusnok's cabinet, and a shows Zeman's strong influence over the party he once used to lead.

The crisis may lead to two more attempts to form a cabinet, or to a cross-party agreement to dissolve parliament and hold an early election. Parties fear Zeman may try to prolong the process, leaving Rusnok's team in power in a caretaker capacity.

(Reporting by Jan Lopatka; Editing by Jason Hovet and Michael Roddy)