By Thomas Escritt

BERLIN (Reuters) - Germany's populist political parties look set to win enough seats to potentially gum up the workings of parliament - even if they don't form part of the next administration. 

HOW POPULAR ARE THEY EXACTLY?

The far-right Alternative for Germany is running second to the conservatives in polls ahead of the national election on February 23.

Even if a late shift or a polling error sees them tying with opposition leader Friedrich Merz's conservative bloc on election night, they are very unlikely to take power: every other party has said they will refuse to govern with them.

But the nativist AfD, which wants to stop arming Ukraine and restore Germany's energy relationship with Russia even as Moscow fights a war of conquest against its western neighbour, may have other ways of throwing a spanner in the works.

Under German electoral law, 20% in the polls might be sufficient to win a quarter or a third of seats in parliament if several parties come close to, but miss the 5% threshold below which they don't win any seats at all.

Alternatively, the left-populist Sahra Wagenknecht Alliance (BSW), which shares the AfD's view of Russia and Ukraine and also opposes increased military spending, could clear the 5% threshold and join forces with it to block legislation.

The AfD already holds a one-third blocking minority in two state parliaments, which gave it the power to disrupt and slow the formation of a new government and leaves regional governments facing a headache with respect to appointing new judges and security officials.

WHAT COULD THE AFD DO WITH A THIRD OF THE VOTES IN PARLIAMENT?

It takes a vote by two-thirds of the members of parliament to initiate a constitutional amendment. That means any bloc that commands a third of the votes can stop a change from happening.

This matters most in the context of Germany's constitutional debt brake - in practice a spending cap that limits the amount by which borrowing can be increased each year. 

Chancellor Olaf Scholz's Social Democrats, Merz's conservatives, and the Greens have said they would be open to reforming the debt brake to increase military spending and support Ukraine given the urgency of the Russian threat to European security.

A blocking alliance of the AfD and BSW would make financing increased military spending all but impossible.

Economists say a reform of the debt brake could, by opening the taps for public spending in one of the world's most frugal industrial nations, be a major growth driver.

Equally, repeated failures to get around its strictures were a major contributor to the collapse, a year ahead of schedule, of Scholz's unwieldy coalition government.

An earlier concern was that the AfD could block the appointment of new judges to the Constitutional Court, gradually undermining the institution that is seen as the main guarantor of Germany's democratic order.

A constitutional amendment, rushed through in the dying days of this parliament precisely out of fear that the AfD would win a blocking minority, has greatly constrained their ability to do this.

The mainstream parties have other plans that would need debt brake reform: the SPD wants to borrow to create an investment fund to reinvigorate the economy, and the conservatives' spending plans might prove impossible to manage without new debt.

WHAT COULD THE AFD OR BSW DO WITH A QUARTER OF VOTES IN PARLIAMENT?

It is far more likely the AfD, possibly in conjunction with the BSW, could win a quarter of the seats.

This would give it the power to set up a parliamentary committee of inquiry, which itself would have the authority to summon witnesses, including state officials.

If parties held a quarter of the seats in a parliamentary committee, they could also vote to make confidential committee proceedings public, creating opportunities to influence the political agenda.

Both the AfD and BSW have long demanded an inquiry into the state's handling of the COVID-19 pandemic, but committees of inquiry could potentially be held on any topic such as support for Ukraine, energy policy, or support for Russia.

WOULD THE AFD AND BSW HAVE AN INTEREST IN WORKING TOGETHER?

The BSW has said it will not work with the AfD, but there is considerable crossover between the two parties' policy agendas and they also share an interest in weakening the political mainstream.

"Both AfD and BSW have very little interest in helping out the government," wrote Eurasia Group analyst Jan Techau in a note. "Their interest lies in doing everything to make the centrist parties look inept and like a spent force."

(Reporting by Thomas Escritt; Editing by Hugh Lawson)