May 12 (Reuters) - An alliance of pro-Western parties led by the Democratic Party won first place in Serbia's general election on Sunday, slightly ahead of the nationalist Radicals.

Results indicated a scramble to clinch a parliamentary majority and a new governing coalition. Here are some scenarios for what could happen next, with the strength of each possible grouping in the 250-seat parliament:

PRO-WESTERN COALITION WITH TACIT OR EXPLICIT NATIONALIST

SUPPORT

SEATS: between 123 and 129

The only way for the Democrats to form a majority government is through an alliance with several ethnic minority parties, and one of two minor partners: the small, ultra-liberal Liberal Democratic Party (LDP) and the Socialists, the once-dominant party of the late Slobodan Milosevic.

A Democrats-LDP coalition would be just shy of a majority. The Socialists have said they cannot formally ally with the LDP -- the only party that says Serbia should accept the secession of Kosovo -- but they could be persuaded to support the coalition in parliament in exchange for concessions.

Political sources say the Democrats would prefer to woo the Socialists into an outright coalition, without the LDP. Such a government would have a thin majority and would probably be tested by disagreements over key issues, such as Kosovo and Serbia's EU future.

THREE-PARTY NATIONALIST COALITION

SEATS: 127

The Radicals floated the idea of an alliance with outgoing Prime Minister Vojislav Kostunica during their election campaign, noting that the premier's opposition to the Western-backed secession of Kosovo made him a natural partner.

If they enlisted the help of the Socialists, the nationalist bloc would have a strong majority in parliament. It would probably put Serbia's bid for European Union membership on ice and have very cool ties with Washington and Brussels. However, they would try to reassure investors that the country is open for business, as foreign investment is key to the economic growth needed to deliver on their populist promises.

TWO-PARTY NATIONALIST COALITION, WITH SOCIALIST SUPPORT

SEATS: 107

Even if the Socialists do not formally join a nationalist coalition, there is a precedent of them supporting Kostunica in parliament, allying themselves with the government in key votes to deliver a majority. Although the partners in such a government would see eye-to-eye in most matters, the Socialists' demands for concessions or key positions for their officials could eventually put pressure on the government.

COALITION OF NATIONALIST WITH PRO-WESTERN PARTIES

SEATS: 132

The Democrats and Kostunica's DSS party were allies in the government that collapsed in March after only eight months in power. There has been no rapprochement on the issue that divided them -- Serbia's response to the West after Kosovo's secession -- and they attacked each other bitterly before the election.

Furthermore, the Democrats appear unwilling to make major concessions in their pro-Western programme to woo Kostunica.

NO COALITION AGREEMENT, NEW ELECTIONS

If no coalition is formed by mid-September, the country will hold a repeat election. Kostunica's outgoing government will be in charge until then but with its mandate severely limited, further delaying important reforms.

(Writing by Ellie Tzortzi; editing by Ralph Boulton)

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