Angela Merkel's re-election as Chancellor of Germany this weekend is almost a foregone conclusion, but putting together a coalition government in her fourth term in office could be the biggest challenge Europe's top politician has ever faced in the region's most-important country.

No party has been able to rule Germany without a coalition partner since 1966, and at this late stage the polls do seem to suggest that Merkel's center-right Christian Democratic Union and its Bavarian sister party the Christian Social Union will win a plurality. Yet this election -- and the coalition that eventually emerges from it - will matter not just to Germany, but to Europe and America and the West, and Merkel's choices of coalition partners remains up in the air.

The steady-as-she-goes option of a continued Grand Coalition with the left-of-center Social Democrats (SDP), led by former European Parliament President Martin Schulz, may no longer be on the table if that party loses votes on Sunday. In any case, much of the SDP base feels damaged by its association with the policies of the CDU and would prefer a period in opposition. But other choices could also have profound effects on policies towards Europe, the environment and even Brexit that are less easy to predict.

Merkel's pragmatic reputation as "Mutti" -- or 'Mom' in German -- is well-earned but her position as the most powerful politician in European Union, and more importantly her recent elevation in the public mind as "the leader of the free world", are to some extent also a product of what she is not.

And that might shape her choice of coalition partners. Her decision to welcome a million refugees in 2015, a rare example of her heart overruling her head, led to a resurgence of anti-immigrant feeling and resentment in Germany that played into the hands of the far-right and xenophobic Alternative for Germany party (AfD).

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Her core belief in the integrity of European Union led her, and her reluctant support for the bailouts of failing Eurozone members in defiance of her hawkish finance minister Wolfgang Schaeuble, boosted the profile of the libertarian Free Democrat Party.

The most recent polls suggest the CDU/CSU bloc is at almost 38%, and her largest rivals, the SPD, are on about 23%. But the AfD and the far left die Linke party, many of whose members are former East German Communists, are in third and fourth place respectively.

Merkel has insisted she will go into coalition with neither, although the AfD will likely make significant gains. Though the comparison with neo-Nazi groups may be unfair, the AfD is often spoken of as the first far-right party likely be represented in the federal parliament since the fall of Adolph Hitler in 1945.

That leaves a choice of a coalition with the economically liberal, but fiscally conservative Free Democrats (FDP) or the environmentally interventionist Greens. The SPD or the Greens might offer tepid support for the separate budget and finance ministry for the Eurozone which French President Emanuel Macron is trying to force on the Germans - something the FDP rejects.

A coalition with the SDP that includes the Greens and FDP would be tough to put together given the former's support for environmental reforms that would be hard to square with the pro-business leanings of the latter. But the numbers may well dictate the outcome.