Berlin  - German Chancellor Angela Merkel's Christian Democrats (CDU) won a regional election Sunday in the German state of Hesse, enabling the party to form a coalition after 12 months of political stalemate.

The centre-left Social Democrats (SPD), who govern at the national level with the CDU, saw their share of the vote plunge to an historic low after narrowly failing to unseat CDU Prime Minister Roland Koch a year ago in a closely contested state election.

Merkel's conservative party was unable to profit from the SPD losses but was helped instead by a strong performance from its preferred coalition partner, the liberal Free Democrats (FDP).

Final returns showed that the CDU polled 37.2 per cent, a minimal improvement from its 36.8 per cent in 2008 in Hesse, while the FDP climbed to 16.2 per cent, gaining 6.8 percentage points from last year, giving the two parties a comfortable majority in the 110-member legislature in Wiesbaden.

Sunday's vote came less than a week after Merkel unveiled a 50- billion-euro (67-billion-dollar) economic stimulus package meant to cushion the effects of a deepening recession.

Analysts said that the Hesse result could affect cooperation between the chancellor and SPD as the CDU seeks to accommodate the FDP as a future national coalition partner.

The vote marked the start of a super-charged election year, which sees five of the country's 16 states go to the polls, as well as elections for a new president in May, European elections in June and a general election on September 27.

FDP national leader Guido Westerwelle called Sunday's results "a decisive signal for the general election," after his party achieved one of its best-ever results.

Hesse, one of Germany's most prosperous states including the financial centre of Frankfurt, has been administered for the last 10 months by a caretaker government led by Koch, 50.

Voter support for the Hesse SPD had steadily eroded since two botched attempts to topple Koch and form a minority government with help from the Greens and the radical Left Party, which has its roots among former East German communists.

Analysts said that voters punished the SPD for a decision by its state leader, Andrea Ypsilanti, to seek the backing of the Left, despite promising in campaign speeches that she would not do so.

Ypsilanti announced her resignation as party chairwoman immediately after Sunday's results showed that the SPD had polled slightly less than 23.7 per cent, a loss of 13 percentage points from a year ago.

The Greens won 13.7 per cent, up from to 7.5 per last year, and the Left Party again narrowly cleared the 5-per-cent hurdle for parliamentary representation.

Only 61 per cent of the 4.4 million eligible voters cast their ballots, a decline of 3.3 per cent and the lowest turnout ever for a state election in Hesse.

Koch was challenged by Thorsten Schaefer-Guembel, 39, an SPD backbencher who is expected to take over the party leadership from his mentor, Ypsilanti.

A hardline member of the CDU's right wing, Koch sold himself as a crisis-manager at a time of growing economic uncertainty. (dpa)