Britain’s deputy prime minister, Nick Clegg, on Friday ruled out supporting a Labour government that also relies on Scottish nationalists for a majority in Parliament.

The Liberal Democrat leader told the Financial Times he would not “help establish a government which is basically on a life support system, where Alex Salmond could pull the plug any time he wants.”

Salmond, the former Scottish first minister, will lead the Scottish National Party in Westminster where it is projected to take as many as 54 out of 59 Scottish seats — an increase of 48 for the party that advocates independence. Given that many of the nationalists’ gains would come at Labour’s expense, the socialist party could probably only form a government with their support.

If Labour does worse in next month’s election than the polls now predict, however, it could need the support of a third party to take power.

Clegg said he wouldn’t be that third party and lamented that Labour had been consumed by “frothing bile” toward the liberals for the past five years.

After the 2010 election, Clegg could have supported Labour but chose to go into coalition with David Cameron’s Conservatives instead.

On Friday, he argued that any coalition with the party that finishes second in the election would lack “legitimacy.”

You cannot provide stability, you can’t take difficult decisions, if people are constantly questioning the birthright of a government.

Cameron is likely to fall short of an outright majority but could emerge with the most seats like last time.

Clegg claims the Liberal Democrats are the only party that can rein in the Conservatives’ worst instincts. He criticized the bigger party’s “socially regressive” plans for spending cuts as well as its “obsession” with Europe which he said undermined British influence in the world.

But the liberals have already accepted that a referendum on European Union membership will take place under another Conservative-led government and the two parties broadly agree on what fiscal policy should look like the next five years.

Both are in favor of raising the income tax threshold to take low-wage workers out of tax. Both want to keep spending down in order to reach a balanced budget within the next few years. Both agree the National Health Service should be exempt from austerity. They even agree “benefit tourism” from other European Union countries must be stopped.

Politically, another coalition also makes sense for leaders in both parties.

Cameron doesn’t want to be beholden to the most reactionary lawmakers in his party who would take Britain out of the European Union and cut back the state to such an extent that it would certainly alienate centrist voters.

Clegg has already lost many of his left-leaning voters to either Labour or the Greens. Joining the SNP in supporting a Labour government would appal his remaining moderate voters in especially the south of England.