Polls opened in the German state of Saarland today in the first of a trio of state elections that could banish Angela Merkel's junior coalition partners from regional parliaments, weakening her centre-right government ahead of next year's federal vote.

The Christian Democrat (CDU) Chancellor plans to seek a third term in 2013, but will almost certainly be forced to find an alternative partner to the liberal Free Democrats (FDP), who languish at just 4% in national polls after a record 14.6% showing at the last national election in 2009.

The CDU is running neck-and-neck with their main rivals, the Social Democrats (SPD), in Saarland, one of Germany's smallest states with just 1 million inhabitants.

The most likely outcome is a "grand coalition" government of the two broadly centrist parties - a result which could be a harbinger of the outcome in next year's federal election.

First results of the Saarland poll are due at 5pm.

The snap vote in the scenic state on the French border was triggered by parochial infighting in the FDP, which brought down the fragile alliance of CDU, Greens and FDP that had governed for the past two years.

The CDU and SPD both scored 34% in the latest survey in Saarland, which has seen its traditional heavy industry and mining replaced by automobile plants and light manufacturing.

It is also the home of leftist leader and former German finance minister Oskar Lafontaine, whose Left party could score 15%. The FDP is expected to win just 2%, well below the 5% threshold required to win assembly seats.

The campaign in Saarland, which formally became part of then-West Germany only in 1957 after a debate on whether it should join France, has focused on local issues like the renewal of state infrastructure and overspending on a new gallery in the regional capital Saarbruecken.

But the contest could set the tone for two crucial regional votes in May - in Schleswig-Holstein on 6 May and North Rhine-Westphalia (NRW), Germany's most populous state, on 13 May.

All three will lay the ground for the national vote next year, offering clues about Dr Merkel's re-election chances and the kind of coalition that will rule in Berlin from 2013.

With 18 million people, NRW is larger than many European countries, and its elections have had a destabilising effect on national politics in recent years.

The ousting of the CDU state premier in NRW in 2010 lost Dr Merkel her majority in the upper house of parliament, making it difficult to pass legislation.

Five years earlier, a humiliating loss for the SPD in NRW, whose big cities and heavy industry had made it a centre-left stronghold for decades, prompted Chancellor Gerhard Schroeder to call early national elections, which hoisted Dr Merkel into power.

A fresh national opinion poll published by Emnid today saw the CDU remaining the most popular party with 35% of the vote, the SPD strengthening by one point to 28%, the Greens at 15%, the Left party and the anti-establishment Pirate party both at 7%, and the FDP at 4%.

A general feeling has developed in Germany that little distinguishes the policies of two biggest parties.

The FDP's young leader, Philipp Roesler, told the Hamburger Abendblatt newspaper in remarks sure to anger Dr Merkel that the CDU was a "social democratic party" forming an ever thicker "mish-mash" with the SPD.

Relations between the coalition partners have been highly strained since the FDP supported the centre-left's choice for German president last month, Joachim Gauck.

If the FDP is ejected from the local assemblies in Saarland, Schleswig-Holstein and NRW, party insiders say that Mr Roesler – also Dr Merkel's economy minister - would be forced to resign.

The FDP failed to secure seats in five state assemblies last year.